chore: use fork of Traceur

This fork contains fix for setters (https://github.com/google/traceur-compiler/pull/919) and also `pureES6` option to compile ES6+ into ES6.

WARNING:
At this point, we only generate shrinkwrap for dependencies (not devDependencies). I manually added a shrinkwrap for `gulp-traceur` and `karma-traceur-preprocessor` so that they both use the traceur fork.
chore: add ES6 build