Preserve file extension on tempfile creation

This was causing scss-lint to skip over our css files, since
Tempfile.new appended stuff onto the end of the file name to guarantee
uniqueness.

Fortunately, they provide a syntax for preserving the extension, which
we now use.

Fixes https://www.pivotaltracker.com/story/show/46567233

Change-Id: I0a87670f6bd06486e54cac487a7f25ef4a91711c
Reviewed-on: https://gerrit.causes.com/19908
Tested-by: Aiden Scandella <aiden@causes.com>
Reviewed-by: Greg Hurrell <greg@causes.com>