Make cache robust to periodic deletions. (#341)

* Add test to check for cached Homebrew dirs - if it hasn't been installed, then do the install from Homebrew.

Author: spco

.travis.yml

@@ -23,8 +23,10 @@
         - numdiff
 
   before_install:
-    - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then export DYLD_LIBRARY_PATH=$PWD/cvode/lib:$PWD/openlibm-0.4.1 ; fi
     - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then rm '/usr/local/include/c++' ; fi
+    # if cached dirs exist, then continue. Otherwise, do the install from Homebrew
+    - if [[ "$TRAVIS_OS_NAME" == "osx" && ! -d /usr/local/Cellar/gcc@4.9/4.9.4_1/bin ]] ; then brew update ; brew install gcc@4.9 libffi gettext ; fi
+    - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then export DYLD_LIBRARY_PATH=$PWD/cvode/lib:$PWD/openlibm-0.4.1 ; fi
     - if [[ "$TRAVIS_OS_NAME" == "osx" ]];   then ./tools/install/install_cvode.sh $PWD /usr/local/Cellar/gcc@4.9/4.9.4_1/bin/gfortran-4.9 ; fi
     - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then ./tools/install/install_cvode.sh $PWD /usr/bin/gfortran ; fi
     - ./tools/install/install_openlibm.sh $PWD