Implement "update" behavior

[sdboyer]

There's no way, at present, for users to update deps within existing constraints, at all. That includes both updating all deps, and updating just specific individual deps. The spec doc includes this functionality under ensure, and has this to say:

For updating all deps at once:

Flags:
	-update		update all packages

For updating just targeted deps:

Fetch/update github.com/heroku/rollrus to latest version, including transitive dependencies (ensuring it matches the constraints of rollrus, or—if not contrained—their latest versions):
	$ dep ensure github.com/heroku/rollrus

I actually asked @peterbourgon to basically cut these out of the improved helptext in #78 because we don't support them right now.

Now, -update is pretty easy. (I might actually just whip up a PR tonight that does it). But I feel a little hesitant about the second one, because now that we're actually writing this, it strikes me as counterintuitive. Having the constraint changes the manifest, but not having it induces an update (assuming it's already in the manifest?). And, if you ensure github.com/foo/bar and it's already present in the project, you get update behavior, but if it's not present, then...what? We've got two identical inputs from the user taken on significantly different intent depending on current state. That feels like we're heading into icky UX territory.

Ofc, maybe that's fine, at this early stage - we can try something, get feedback, then trying something else.

[jessfraz]

closing as #84 is merged