Specify sass version

Hi!

I've found that `sass 3.3.x` breaks some of the css related specs of jsduck.
Although the build on ruby 1.8.7 is still failing, this change fixes the builds on ruby 1.9.3 and 2.0.0 on Travis-ci.
( https://travis-ci.org/kt3k/jsduck/builds/25392475 )

Thanks!