Use the last version in the 3.0.x releases as the default.

This patch uses v3.0.21 of passenger, the last in the 3.0.x branch as
the default instead of 3.0.9, which was likely just the newest at the
time the module was created.
Merge pull request #58 from ghoneycutt/use_passenger_3_0_21

Use the last version in the 3.0.x releases as the default.