feat(slugbuilder): build without using a pipe for git archive

Ref #785
Ref deis/slugbuilder#9

This will break builds if the corresponding slugbuilder patch is not
merged first.
Merge pull request #1641 from deis/fix/builder/no-pipe

feat(slugbuilder): build without using a pipe for git archive
feat(*): add `make test-style` targets

This gives us an opporunity to run code checkers and linters as part
of testing, as we used to on travis-ci.org. By default, unit tests
will check code syntax and style first.