Add Qt5 bin path to travis.yml

homebrew qt5 is keg-only, it doesn't create symlinks, so we either
need to add to $PATH or create symlinks. Let's try the former for now.

.travis.yml

@@ -21,6 +21,7 @@ before_script:
        clang++ -E -x c++ - -v < /dev/null; \
        export CPLUS_INCLUDE_PATH=/usr/local/include; \
        export LIBRARY_PATH=/usr/local/lib; \
+       export PATH="/usr/local/opt/qt/bin:$PATH"; \
      fi"
 
 addons: