travis: Enable ccache support

This should halve build times in the best case, and only seems to expose
one GCC warning-related issue that will be addressed by the next commit.
Actual build times will continue to vary wildly based on the kind of
changes people push and the build's I/O-bound nature triggering the
limitations inherent to shared hosting services.

.travis.yml

@@ -1,5 +1,6 @@
 language: cpp
 sudo:     required
+cache:    ccache
 
 services:
     - docker