[rtl] Fix rtl specific variables not overrided in Sass version

Hello Antoine,

The Sass version of Tetra-UI doesn't render the right-to-left (RTL) same as Less version, it's due to the application of a series of "!default" into "variables_rtl.scss" variables (which is the specific override file for RTL), this has no effect to overrides.

For fixing this, I introduce a new Grunt task named "string-replace:less2sass_variables_override" which not add the "!default" token for each copied variable, this "force" the value.

Thanks a lot for the integration.