Replace versionInWorkspace() stub

[sdboyer]

In #18, we introduced a versionInWorkspace() stub function that is supposed to inspect a GOPATH workspace and determine the version of a provided project root. Right now, we've just hardcoded versions for the dep tool's own external deps, but this needs to be actually addressed.

@freeformz said he might be able to take a stab at this, so I'm assigning him.

[sdboyer]

I do have one note to raise...

@freeformz, when you mentioned that you had some existing code to do stuff like this in godep, the thought occurred to me that gps already relies on Masterminds/vcs for its basic repo interactions. That lib isn't perfect (what is), but I am a little worried about having different vcs interaction layers used in different parts of the program. Then again, if we generally agree that's unwise in the long term, then I also don't really want to block progress - I think it might be OK to pull in what you have so that we have this problem solved, with the intention that we refactor later?