upgraded macports build to 10.11

this required a few fixes:
- use export COLUMNS instead of stty
- install flex +universal

The latter is needed for some misterious reason. I think
it is a bug in the libmatheval port file

.travis.yml

@@ -52,9 +52,11 @@ matrix:
     env: PLUMED_CC=mpicc PLUMED_CXX=mpic++  PLUMED_CXXFLAGS=-O3
 # osx serial macports
   - os: osx
+    osx_image: xcode7.3
     env: PLUMED_MACPORTS="plumed"
 # osx serial macports debug variant
   - os: osx
+    osx_image: xcode7.3
     env: PLUMED_MACPORTS="plumed +crystallization +manyrestraints"
 # I don't put any mpi variant with macports since it takes too long to compile
   allow_failures:
@@ -105,9 +107,13 @@ install:
 # I do it only when LD_LIBRARY_PATH is non blank, since otherwise clang gives problems
   - if test -n "$LD_LIBRARY_PATH" ; then PLUMED_LDFLAGS="-Wl,-rpath,$LD_LIBRARY_PATH" ; fi
 # macports related stuff
-  - if test -n "$PLUMED_MACPORTS" ; then stty rows 40 columns 80 ; fi
-  - if test -n "$PLUMED_MACPORTS" ; then wget https://distfiles.macports.org/MacPorts/MacPorts-2.3.4.tar.bz2 && tar xvfj MacPorts-2.3.4.tar.bz2 ; fi
-  - if test -n "$PLUMED_MACPORTS" ; then cd MacPorts-2.3.4 && ./configure && sudo make install && cd - && sudo rm -fr MacPorts-2.3.4 ; fi
+  - if test -n "$PLUMED_MACPORTS" ; then export COLUMNS=80 ; fi
+# install macports from source:
+#  - if test -n "$PLUMED_MACPORTS" ; then wget https://distfiles.macports.org/MacPorts/MacPorts-2.3.4.tar.bz2 && tar xvfj MacPorts-2.3.4.tar.bz2 ; fi
+#  - if test -n "$PLUMED_MACPORTS" ; then cd MacPorts-2.3.4 && ./configure && sudo make install && cd - && sudo rm -fr MacPorts-2.3.4 ; fi
+# install macports from binary (faster):
+  - if test -n "$PLUMED_MACPORTS" ; then wget https://distfiles.macports.org/MacPorts/MacPorts-2.3.4-10.11-ElCapitan.pkg ; fi
+  - if test -n "$PLUMED_MACPORTS" ; then sudo installer -pkg MacPorts-2.3.4-10.11-ElCapitan.pkg -target / ; fi
   - if test -n "$PLUMED_MACPORTS" ; then export PATH=/opt/local/bin:$PATH ; fi
   - if test -n "$PLUMED_MACPORTS" ; then sudo port -v selfupdate ; fi
   - if test -n "$PLUMED_MACPORTS" ; then make macports ; fi
@@ -127,6 +133,8 @@ script:
   - if test "$PLUMED_CXX" ; then make install prefix="$HOME/opt" ; fi
 
 # build using macports
+# this is required for some misterious reason:
+  - if test -n "$PLUMED_MACPORTS" ; then sudo port install flex +universal ; fi
   - if test -n "$PLUMED_MACPORTS" ; then sudo port install $PLUMED_MACPORTS ; fi
 
 # TEST: