Upgrade to ember-cli-babel >=6.6

[gabrielgrant]

BUG REPORT

Summary

Addon uses ember-cli-babel 5.2.8, which is deprecated.

Expected Behavior

no warnings

Actual Behavior

Installing shows the following:

$ ember install ember-spread
WARNING: Ember CLI is using the global npm, but your npm version has not yet been verified to work with the current Ember CLI release.
npm: Installed ember-spread
DEPRECATION: ember-cli-babel 5.x has been deprecated. Please upgrade to at least ember-cli-babel 6.6. Version 5.2.8 located: ff -> ember-spread -> ember-cli-babel
DEPRECATION: ember-cli-babel 5.x has been deprecated. Please upgrade to at least ember-cli-babel 6.6. Version 5.2.8 located: ff -> ember-spread -> ember-prop-types -> ember-cli-babel
DEPRECATION: ember-cli-babel 5.x has been deprecated. Please upgrade to at least ember-cli-babel 6.6. Version 5.2.8 located: ff -> ember-spread -> ember-prop-types -> ember-get-config -> ember-cli-babel
DEPRECATION: ember-cli-babel 5.x has been deprecated. Please upgrade to at least ember-cli-babel 6.6. Version 5.2.8 located: ff -> ember-spread -> ember-prop-types -> ember-string-ishtmlsafe-polyfill -> ember-cli-babel
Installed addon package.

Steps to Reproduce

install on ember v3.1