refactor(scripts): Use Karma CLI rather than grunt (#137)

## Summary

- Drop `grunt` and related dependencies

Every Karma config option has a corresponding CLI option, which takes precedence over any option of the same name in the config file. We don't need to use grunt to run these tests. Additionally, I'm pushing the boundaries of the oldest Node.js versions we can run in CI, and `grunt` was a [hangup](https://travis-ci.org/optimizely/javascript-sdk/jobs/398453541) for doing validation with 0.10.44.

## Test plan
CI.

Author: spencerwilson-optimizely