Travis speedup by using ccache

[pmconrad]

Full run with prepped cache ~25 minutes, that's 50% of a normal run. https://travis-ci.org/bitshares/bitshares-core/builds/417273045

Downside is that builds with an empty or outdated cache may skip tests and/or sonar scanner run, in order to make sure the updated cache is preserved by travis. Manual restart should result in a full run with prepped cache in this case.

[abitmore]

$ [ ! -r _empty_cache ] || ( echo "Please restart with populated cache" && false )
Please restart with populated cache
The command "[ ! -r _empty_cache ] || ( echo "Please restart with populated cache" && false )" exited with 1.

Retrying.

[abitmore]

Oh, it's solved by restarting the build (as intended). Thanks.