Add Sass 3.3.0 dependency to README

We get a question every once in a while about `scss-lint` not working
for an older version of Sass. Hopefully explicitly mentioning this will
reduce the number of inquiries.

Change-Id: Ic761ddec1183ecab1da5b70fbe7dc1a0cbd9a4bf
Reviewed-on: http://gerrit.causes.com/36179
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>