Update mail to minimum version 2.5.4
Bump rack dependency to 1.4.5
Enforce i18n version >= v0.6.4

v0.6.2 and v0.6.3 both have issues with Ruby 1.8.7, see
https://github.com/svenfuchs/i18n/issues/192 for more info.
Merge rails 3.2.16 gemspec changes into our fork