Fix double comma in selector

In make-grid-columns() and make-grid-columns-float() mixins there was a double comma in the list of selectors ($list). This was find for Ruby sass compiler, but libsass didn't accept it.

Fixes issue #117.
fixes missing grid column reset classes
Merge pull request #119 from dchest/patch-1

Fix double comma in selector
Merge pull request #126 from pburgmer/master

fixes missing grid column reset classes
update with latest converter.rb improvements

by @glebm - thx again
build bootstrap v3.0.2 with sass version 3.2.12

`rake build`