Fixes Anvil support of local build pack

Right now the `@buildpack` var is nil which causes Anvil to default to the production build pack, this change tells Anvil that we want to build the build pack in the current directory. 

Anvil deploys are now tested
Merge pull request #4 from heroku/schneems/fix-and-test-anvil

Provide sane defaults for Anvil buildpack settings