travis-ci/osx: install gcc and fix CC

Since there is no gcc in osx_image [1]:
    ls: /usr/local/bin/gcc-?.?: No such file or directory

  [1]: https://travis-ci.org/libevent/libevent/jobs/484794192#L95

And use ls(1) over echo(1) to show an error if there is no such file.

.travis.yml

@@ -50,7 +50,7 @@ before_install:
   - export TIMEOUT=50
   - if [ "$TRAVIS_OS_NAME" == "osx" ]; then
       if [ "$CC" == "gcc" ]; then
-        export CC=$(ls -t /usr/local/bin/gcc-?.?);
+        export CC=$(ls /usr/local/Cellar/gcc/*/bin/gcc-?);
       fi
 
       export OPENSSL_ROOT=$(echo /usr/local/Cellar/openssl/*);
@@ -82,6 +82,7 @@ addons:
     - openssl
     - lcov
     - libtool
+    - gcc
 
 
 script: