Lock major-version, track minor-version of dependencies

Sync up to source-map 0.1.34.
Improve source map accuracy.

Review URL: https://github.com/google/traceur-compiler/pull/1140

Closes #1140.
separate AlphaRenamer from ScopeTransformer base class
Report ModuleEvaluationError via .message.
Unfortunately seems like Node prints Error.message rather than Error.toString().