Complete `get_minimum_required()` algorithm

[schlessera]

The algorithm behind get_minimum_required() is still missing, it only fetches bare version. For a requirement like '>5.4.2', this will return incorrect values.

This can probably be easily done once #1 has been taken care of.

[schlessera]

This is good enough as of v0.2.5 to be usable, but a real PHP5.2 compatible SemVer library should be used for 1.0.0.