Deal with buildpacks without default_process_types

Although the buildpack API document
(https://devcenter.heroku.com/articles/buildpack-api#bin-release) seems
to suggest everything will have default_process_types key looking at
lots of the third party buildpacks they don't
(https://devcenter.heroku.com/articles/third-party-buildpacks)

This patch ensures that the merge on line 134 (now 136) doesn't throw an
exception because default_process_types is Nil, which happens for
example with this buildpack: https://github.com/ddollar/buildpack-test
Merge pull request #16 from garethr/master

Deal with buildpacks without default_process_types