Use brew link to install gcc in a more predictable place

.travis.yml

@@ -20,11 +20,12 @@ matrix:
         homebrew:
           packages:
             - astyle
-            - gcc
+            - gcc@8
       before_install:
         - pip3 install -r requirements.txt
+        - brew link gcc
         - export PATH="/usr/local/bin:$PATH"
-        - ln -s /usr/local/Cellar/gcc/8.3.0/bin/gcc-8 /usr/local/bin/gcc
+        - ln -s /usr/local/bin/gcc-8 /usr/local/bin/gcc
         - gcc --version
       script:
         - "cd test && python3 -m nose --rednose --verbose"