build: don't let libtool insert rpath into binaries
build: Find the proper xcb/pcre dependencies
build: add WIP dependency builder.

This is in constant flux and is not ready for any real use.
travis: initial config
gitian: replace descriptors
travis: use beta builder with cache enabled

Linux only for now, since sudo can't be used
travis: commit to see if cache is activated
travis: attempt to add ccache
Merge pull request #1 from coryfields/depends-travis-cached

Pull-tester depends