Merge pull request #388 from ember-cli/dependabot/npm_and_yarn/ember-source-3.15.0

Bump ember-source from 3.13.3 to 3.15.0

Author: Robert Jackson

package.json

@@ -72,7 +72,7 @@
     "ember-maybe-import-regenerator": "^0.1.6",
     "ember-qunit": "^4.5.1",
     "ember-resolver": "^5.3.0",
-    "ember-source": "~3.13.3",
+    "ember-source": "~3.15.0",
     "ember-source-channel-url": "^2.0.1",
     "ember-try": "^1.3.0",
     "eslint": "^6.6.0",