travis: switch to 'cargo' cache (caches all of target/), and clear valgrind folders before caching
travis: switch to 'cargo' cache (caches all of target/), and clear valgrind folders before caching

imazen/imageflow,imazen/imageflow,imazen/imageflow,imazen/imageflow,imazen/imageflow,imazen/imageflow,imazen/imageflow