Update dependency gulp-eslint

Update `gulp-eslint` from `^2.0.0` (currently `v2.1.0`) to `^3.0.1` and
remove `babel-eslint` which is no longer necessary.

Changes:
- Update eslint from `^2.13.1` to `3.11.1`
- Update the `.eslintrc` config file.

There is no relevant package change.
See http://eslint.org/docs/user-guide/migrating-to-3.0.0