Always require mime-types < 2.0 in tests

This only changes things for Rails 2.3 currently. As we can't properly
define bundle groups for jruby, we will always require
mime-types < 2.0 for now to please the builds. As we don't touch them,
it shouldn't make a difference anyway.
Workaround missing ngigits parameter to Float#round in Ruby 1.8.7
Use smaller float for request duration in tests