Build Erlang Dialyzer PLTs in a separate optional step

It would be cool if the Travis boxes had PLTs prebuilt as doing that per-build is just a waste of time and CPU cycles.

However, since that's not the case at the moment, I've cleaned up the PLT generation a bit in order to prepare them for fetching from external storage during a build. The PLTs have absolute paths embedded so generating in a different env than they are intended to be used in is not an option.

This is my first encounter with Chef, so please shout if anything here smells.