Browser Link: Fixed NullReferenceException
srv-rtlcss: Prevent from throwing.
The problem is, when both rtlcss and autoprefixer
are enabled, it works fine with SCSS, but throws
with LESS.

When the code is too complicated,
the `result` object seems to have problem with
returning `result.css` member. When the code is
comparatively straight-forward, it throws a
slightly lenient error.

/cc @MohammadYounes
Merge pull request #1523 from am11/master

srv-rtlcss: Prevent from throwing
Browser Link: Fixed NullReferenceException
JSON: Check for new schemas every 3 days
Remove redundant autoprefixer call
Stringify prev source maps
Merge pull request #1525 from MohammadYounes/f1

Possible fix for error described in PR#1523
BrowserLink: Removed CSS sync

This will be in the box in VS2013 Update 4