Retrieve more versions instead of only newest one

[Zlopez]

Currently the anitya is retrieving only version that is considered newer than already obtained versions.
It will be nice if it retrieved more versions than just the newest one.
And compare them to already obtained versions and add only the missing ones.

This will help prevent the issue when it is retrieving incorrect version.
See gbrainy project for example:
There is version 2.3.5 available, but when doing test check version 2.06 is retrieved as newest.

[Zlopez]

This is relevant to issue #269

[Zlopez]

This enhancement is merged. Closing.