OSX + homebrew + MPI constantly failes on travis, I make it not compulsory

.travis.yml

@@ -89,6 +89,8 @@ matrix:
   allow_failures:
 # I allow this to fail temporarily
   - env: PLUMED_DOCKER=yes
+# this is failing due to problems in installing MPI, should be allowed to fail until we fix it.
+  - env: PLUMED_CC=mpicc PLUMED_CXX=mpic++  PLUMED_CXXFLAGS=-O3
 ## Possible additional variables:
 #   VALGRIND=yes to make valgrind tests, only when log contains string [valgrind]
 install: