Fixes #537 (charAt problem)

The problem stems from twitter-bootstrap 2.3.1 using new features in Less that require a version greater then 1.3.0. Since twitter-bootstrap-rails depends on less-rails which uses less.rb and is still using lessc 1.3.0, we get this error.

Changing the mixins.less file to use the old less syntax fixes this problem.

This pull request also removes sprites.less again from the default bootstrap.less file since twitter-bootstrap-rails has the option to choose between fontawesome or the glyphicons sprite.