Update travis config to use updated versions of OS and compilers. (#845)
Update travis config to use updated versions of OS and compilers. (#845)

This fixes the linux/gcc error building Google Test with gcc 4.7:

https://travis-ci.org/github/jbeder/yaml-cpp/jobs/668233706
WrinklyNinja/yaml-cpp,dand-oss/yaml-cpp,jbeder/yaml-cpp,dand-oss/yaml-cpp,loot/yaml-cpp,graehl/yaml-cpp,jbeder/yaml-cpp,graehl/yaml-cpp,jbeder/yaml-cpp,WrinklyNinja/yaml-cpp,dand-oss/yaml-cpp,WrinklyNinja/yaml-cpp,loot/yaml-cpp,jbeder/yaml-cpp,dand-oss/yaml-cpp,graehl/yaml-cpp,dand-oss/yaml-cpp,graehl/yaml-cpp,loot/yaml-cpp,WrinklyNinja/yaml-cpp,loot/yaml-cpp,graehl/yaml-cpp,jbeder/yaml-cpp