travis: use ccache as a caching mechanism
travis: use ccache as a caching mechanism

This mechanism is natively supported by travis:

  https://docs.travis-ci.com/user/caching/

This will add /usr/lib/ccache to the front of $PATH.

deepstreamIO/deepstream.io-client-cpp,deepstreamIO/deepstream.io-client-cpp,deepstreamIO/deepstream.io-client-cpp,deepstreamIO/deepstream.io-client-cpp