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

dryproject/libcpr,dryproject/libcpr,dryproject/libcpr,dryproject/libcpr,dryproject/libcpr,dryproject/libcpr