Try and fix Travis CI on OS X

On Linux we have to export CXX=g++-4.8 or it will use Travis's super old g++
and Oniguruma regex engine used by the highlighter won't build. Try and detect if
there is a g++-4.8 binary and if not, fall back to just plain g++

Author: samcv

.travis.yml

@@ -39,7 +39,7 @@ install:
 
 script:
   - make test
-  - export CXX=g++-4.8
+  - export CXX=$(command -v g++-4.8 >/dev/null 2>&1; if [ $? -eq 1 ]; then printf "g++"; else printf "g++-4.8"; fi)
   - $CXX --version
   - make html
 matrix: