Do not update spec repo when passing in a URL

Only call `update_specs_repos` when using a named Pod.

Also, temporarily removed support for `git@github` style URLs until I
can test that without requiring the github.com ssh key added to the
travis environment - as seen in
https://travis-ci.org/CocoaPods/cocoapods-try/jobs/20141514

Any feedback on how best to test this? Should I be using VCR so it
doesn’t actually call out to the server?