Ensured that Travis CI builds use GCC 4.8 instead of GCC 4.6.

This fixes build errors due to old releases of libstdc++ missing
methods added in C++11, such as in particular std::string#pop_back().

See: https://travis-ci.org/dryproject/libcpr/jobs/19689729
See: https://travis-ci.org/dryproject/libcpr/jobs/19689730