polyfill es6.number for IE11

Change-Id: Ib7d0ea906f670f89b767cae0500d05fa03734535
Reviewed-on: https://gerrit.instructure.com/153440
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
Reviewed-by: Jeremy Neander <jneander@instructure.com>
Tested-by: Jenkins
Product-Review: Derek Bender <djbender@instructure.com>
QA-Review: Derek Bender <djbender@instructure.com>

gulpfile.js

@@ -146,7 +146,8 @@ function makeIE11Polyfill () {
     'es6.string.includes',
     'es6.string.starts-with',
     'es6.symbol',
-    'es6.map'
+    'es6.map',
+    'es6.number'
   ]
 
   return coreJsBuilder({