Fix broken R test: Install Homebrew GCC (#4142)

* Fix broken R test: Install Homebrew GCC

Missing GCC Fortran causes installation failure of a dependency package
(igraph)

* Register gfortran system-wide

* Use correct keg

* Set env vars to change compiler choice

* Do not break other Mac builds

* Nuclear option: symlink gfortran

* Use /usr/local/bin instead of /usr/bin

* Symlink library path too

* Update run_test.sh

.travis.yml

@@ -68,6 +68,7 @@ addons:
       - g++-7
   homebrew:
     packages:
+      - gcc@7
       - graphviz
     update: true
 

tests/travis/run_test.sh

@@ -87,6 +87,12 @@ if [ ${TASK} == "r_test" ]; then
     export _R_CHECK_TIMINGS_=0
     export R_BUILD_ARGS="--no-build-vignettes --no-manual"
     export R_CHECK_ARGS="--no-vignettes --no-manual"
+    if [ ${TRAVIS_OS_NAME} == "osx" ]; then
+        # Work-around to fix "gfortran command not found" error
+        sudo ln -s $(which gfortran-7) /usr/local/bin/gfortran
+        sudo mkdir -p /usr/local/gfortran/lib/gcc/x86_64-apple-darwin15
+        sudo ln -s /usr/local/lib/gcc/7 /usr/local/gfortran/lib/gcc/x86_64-apple-darwin15/6.1.0
+    fi
 
     curl -OL http://raw.github.com/craigcitro/r-travis/master/scripts/travis-tool.sh
     chmod 755 ./travis-tool.sh