Add babel-register plugin
Add babel-register plugin

This prevents our gulpfile from printing out an error trying to
require babel-register before falling back to babel-core/register.
This is now possible because we are on Babel 6.

closes gh-867

Test Plan:
  - Run compile assets
  - At the end during the 'rev' step, you should not see:
    ```
       Failed to load external module babel-register
       Requiring external module babel-core/register
    ```
  - Instead you should just see:
    ```
       Requiring external module babel-register
    ```

Change-Id: I3168dbdfb733979e762537f40fdb96effbefa328
Reviewed-on: https://gerrit.instructure.com/85571
Tested-by: Jenkins
Reviewed-by: Steven Burnett <a0b450117c5d20a22d5c4d52bb8ae13adee003d8@instructure.com>
Product-Review: Clay Diffrient <9dff2e5c98626d20d2848250c411e8009465bb87@instructure.com>
QA-Review: Clay Diffrient <9dff2e5c98626d20d2848250c411e8009465bb87@instructure.com>

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