Re-eneabled tests

cc @HanatoK I fixed the problem. Namely, plumed was linking by mistake
another boost version (homebrew).

regtest/drr/rt-extract/config

@@ -2,14 +2,3 @@ type=plumed
 plumed_needs=boost_serialization
 plumed_modules=drr
 arg="drr_tool --extract test.drrstate -v"
-
-# Skip this since it crashes on travis-ci
-# I'm not sure about the reason, could be the boost version of macports
-function plumed_custom_skip(){
-  if [ "$(uname)" == "Darwin" ] ; then
-    return 0
-  else
-    return 1
-  fi
-}
-

regtest/drr/rt-merge/config

-function plumed_custom_skip(){
-  if [ "$(uname)" == "Darwin" ] ; then
-    return 0
-  else
-    return 1
-  fi
-}
 type=plumed
 plumed_needs=boost_serialization
 plumed_modules=drr

regtest/drr/rt-restart/config

@@ -2,10 +2,3 @@ type=driver
 plumed_needs=boost_serialization
 plumed_modules=drr
 arg="--plumed plumed.dat --trajectory-stride 1 --timestep 0.005 --ixyz ala12_trajectory.xyz --dump-forces forces --dump-forces-fmt=%10.6f"
-function plumed_custom_skip(){
-  if [ "$(uname)" == "Darwin" ] ; then
-    return 0
-  else
-    return 1
-  fi
-}