Remove redundant rb-readline from Gemfile

MSP-11585

The entry in the gemspec is sufficient.
Don't depend on 'rb-readline' on Windows

MSP-11585

`rb-readline` gem doesn't work with Ruby Installer for 1.9.3 because of
`NoMethodError undefined method 'dlopen' for Fiddle:Module`.  Removing
`rb-readline` may be a temporary fix it if turns out that this is a bug
with rb-readline or some other upstream bug.
Land #4149, readline adjustments

* Windows guard, DRYing