fix(Makefile): remove bashism in the main Makefile

Mac OSX's make has support for `[[`, Ubuntu's does not. This fix (as
suggested and written up by @carmstrong on a comment[1]) removed the
bashism in favor of a more compatible syntatic trick.

[1]: https://github.com/deis/deis/pull/1380#issuecomment-49632570

Amended to fix a "$$" in place of a "$"