Fixes the reading of dependencies that have spaces in their subspecs' name from Lockfiles

Closes https://github.com/CocoaPods/CocoaPods/issues/2850.
Should've been done with https://github.com/CocoaPods/Core/pull/179.

See http://regexr.com/39uen.
Yeah but if this leads to a complex and un-debuggable `RegExp` that nobody understands, whereas a simpler and more understandable `RegExp` could have done the job, that's too bad.

Besides, that change does not only change this, it also changes the group capturing, which makes things more confusing and doesn't add anything useful to the `RegExp` as it is used only for `match` and not for any `gsub` substitution.