Temporarily changed failing regtest

This regtest is failing on travis from time to time, in an
irreproducible way. I took note of the problem at issue #183

I temporarily change this regtest by:
1. moving COLVAR.reference to COLVAR.reference.dontcheck
2. adding HILLS.reference so that at least one file is checked.

Notice that this is not a solution of the problem, just a way
to avoid random failures on travis.

As soon as someone is willing to fix this, just put COLVAR.reference
back in place and push it to travis

regtest/basic/rt67-mpi/HILLS.reference

+#! FIELDS time phi psi sigma_phi sigma_psi height biasf
+#! SET multivariate false
+#! SET min_phi -pi
+#! SET max_phi pi
+#! SET min_psi -pi
+#! SET max_psi pi
+                      1     -1.483864797032211      1.048215588851982                    0.2                    0.2      1.333333333333333                     10
+                      2     -1.324349464027241     0.6054782610633875                    0.2                    0.2      1.328866944925153                     10
+                      3     -1.334014372833683     0.6807921585013936                    0.2                    0.2       1.25940590072867                     10
+                      4     -1.461334733547937      1.392061395607319                    0.2                    0.2       1.31727463102725                     10
+                      5     -1.220227832598179     0.7871462158259384                    0.2                    0.2      1.233392551193818                     10
+                      6     -1.388275748836901      1.000526280642839                    0.2                    0.2      1.213649736190069                     10
+                      7     -1.548083770068421       1.34534850785474                    0.2                    0.2       1.24072382882099                     10
+                      8     -1.842857738569458      1.329332035812337                    0.2                    0.2      1.294171541970602                     10
+                      9     -2.242442614461024      2.605939458506299                    0.2                    0.2      1.333333333333333                     10
+                     10     -1.148221252978592     0.5350155021525413                    0.2                    0.2      1.228215634730521                     10
+                     11     -1.757993684553651      2.075151176709721                    0.2                    0.2      1.333263095744202                     10
+                     12     -1.318567054245503      3.099684225538316                    0.2                    0.2      1.333333333333333                     10
+                     13     -2.991104158561357      2.899067264217869                    0.2                    0.2      1.333333333333333                     10
+                     14     -1.411190277910799   0.002776825173235409                    0.2                    0.2      1.331643493699761                     10
+                     15      -2.59947290674873      2.668260735688806                    0.2                    0.2      1.314283687641334                     10
+                     16     -1.460820764733805     0.2621640104513725                    0.2                    0.2      1.277086196944669                     10
+                     17     -1.379146894998496      1.157586759697931                    0.2                    0.2      1.164032437271471                     10
+                     18     -1.677124361971585     0.9078208791514708                    0.2                    0.2      1.241906654007741                     10
+                     19     -1.524070841192655      1.262330227864704                    0.2                    0.2      1.106699449165534                     10
+                     20     -1.199652274777625     0.9529133238131449                    0.2                    0.2      1.156702641073293                     10