Make bin/build self-contained
Make bin/build self-contained

This commit makes the build script usable for distributing the
executable, e.g. from a Homebrew formula.

- It works from any directory, including outside of the GOPATH
- Dropped the dependency on godep and hence hg

Signed-off-by: Max Brunsfeld <8ee73e75b50cc292b5052b11f2ca25336d3e974e@pivotallabs.com>

cloudfoundry/cli,cloudfoundry/cli,cloudfoundry/cli