Pin source-map version to 0.1.33 to avoid bug introduced in 0.1.34

Bug causes bad sourcemaps to be generated when there are consecutive
newline characters in files.

See: https://github.com/mozilla/source-map/pull/116

This pin should be removed when source-map 0.1.35 is released.
Merge pull request #15 from rianmcguire/master

Pin source-map version to 0.1.33 to avoid bug introduced in 0.1.34