Make sure we are comparing gem versions, closes #1024

Due to rails/rails#8501, we can no longer case the Rails version to a float. This version should hopefully work on both Rails 4 master and on Rails 3.
Replace ugly regex with POSIX class

`[[:space:]]` works like `\s` in regexes, except it is encoding aware.
tagged 2.0.3
Merge commit '2.0.3' into capybara-2