Specify sass version
Merge pull request #559 from kt3k/feature/fix-travis-build

Specify sass version.

Because 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.
Fix typo in built-in class docs.

Fixes #577
Merge branch 'master' into troopjs