Use pessimistic operator in README.txt

I propose changing the Gemfile operator in the README from `>=` to `~>`.

Bootstrap changes significantly from version to version.  When, for example, Bootstrap 4 comes out, anyone who is using `>=` and runs `bundle update` will have issues.

For further argument in favor of the pessimistic operator please see:
http://yehudakatz.com/2010/08/21/using-considered-harmful-or-whats-wrong-with/