Sasha Case

I couldn't get this working due to: ``` python: can't open file 'virtualenv.py': [Errno 2] No such file or directory ``` Inspecting the source reveals this is now located inside...

Add --prefer-locked-dependencies option to upgrade command

15

**Summary** When upgrading a dependency, the user does not always want to also update all transitive (sub) dependencies. For example, when locking down or stabilising a system towards the end...