Add GitSpecification#pretty_print
Add LockSpecification#pretty_print
Test sort order for installed source

When compared to git and vendor sources there were no tests.

Part of #944
Test vendor vs git source ordering

Part of #944
Prefer git sources over installed sources

This allows a gem from a git repository to be used instead of an
installed gem with the same version.  Without this the installed
version will be used which prevents the user from using the changes
from git which will probably be newer.

Fixes #944