Fix RubyGems 2.0 compatibility

Hi,

this patch should resolve the incompatibility with new RubyGems 2.0 that has already been filed by @voxik  [1].

The main difference is in abandoning of `Gem::Format` in favour of `Gem::Package` which is also the reason why I changed the local variable `format` to `package` in templates.

[1] https://github.com/lutter/gem2rpm/issues/21