Auto merge of #743 - nabijaczleweli:chore/travis-cache, r=kbknapp
Auto merge of #743 - nabijaczleweli:chore/travis-cache, r=kbknapp

Cache cargo on Travis to (hopefully) speed up builds

clap-rs/clap,kbknapp/clap-rs,kbknapp/clap-rs,clap-rs/clap,clap-rs/clap