Merge pull request #37 from rehno-lindeque/pcre8.35

Upgrade pcre to 8.35
Merge pull request #38 from rehno-lindeque/fixffilibs

Fix paths libffi files are being copied to
Clean source downloads from build cache

A big cache slows down subsequent builds
Allow external cache retrieval
Document using Anvil with EXTERNAL_CACHE
Merge pull request #45 from begriffs/external_cache

Apply Anvil cache to pushes
Merge branch 'master' of github.com:begriffs/heroku-buildpack-ghc