Ensured that Travis CI builds always use a newer libstdc++.
Ensured that Travis CI builds always use a newer libstdc++.

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/20654745

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