Ensure we are properly testing multiple Ember versions.

Prior to this commit (since c2ca068) we were only testing ember-source@2.11.
Even though we had a `config/ember-try.js` with many scenarios, none of them
properly removed the `ember-source` version (even though ba7f462 was
specifically trying to do this 😭).

The fundamental issue here is:

* `ember-try` supports removing packages, but requires that you specify them
  in the `dependencies` or `devDependencies` objects.  Since my changes in
  ba7f462 added `"ember-source": null` to the root of the `npm` config
  it was completely ignored 😞.
* `ember-source` emits a warning to the console when it is present **and**
  the bower package for `ember` is in use.  Unfortunately, this warning is
  a bit misleading (it doesn't tell you that the `ember-source` from `npm`
  will always win). The `ember-source` addon should be updated to throw an
  error when this misconfiguration is detected.
* `ember-cli` will **always** use `ember-source` if it is present, and
  completely ignore whatever is in `bower.json`. IMHO, this default was not
  correct and `ember-cli` should have done the same thing that `ember-data`
  did during its migration from bower to `npm`: if both packages are present
  default to `bower.json` version with a warning.

config/ember-try.js

@@ -3,9 +3,8 @@ module.exports = {
   scenarios: [
     {
       name: 'default',
-      bower: {
-        dependencies: { }
-      }
+      bower: { },
+      npm: { }
     },
     {
       name: 'ember-1-13',
@@ -18,7 +17,9 @@ module.exports = {
         }
       },
       npm: {
-        'ember-source': null
+        devDependencies: {
+          'ember-source': null
+        }
       }
     },
     {
@@ -32,7 +33,9 @@ module.exports = {
         }
       },
       npm: {
-        'ember-source': null
+        devDependencies: {
+          'ember-source': null
+        }
       }
     },
     {
@@ -46,7 +49,9 @@ module.exports = {
         }
       },
       npm: {
-        'ember-source': null
+        devDependencies: {
+          'ember-source': null
+        }
       }
     },
     {
@@ -60,7 +65,9 @@ module.exports = {
         }
       },
       npm: {
-        'ember-source': null
+        devDependencies: {
+          'ember-source': null
+        }
       }
     },
     {
@@ -74,7 +81,9 @@ module.exports = {
         }
       },
       npm: {
-        'ember-source': null
+        devDependencies: {
+          'ember-source': null
+        }
       }
     },
     {
@@ -88,7 +97,9 @@ module.exports = {
         }
       },
       npm: {
-        'ember-source': null
+        devDependencies: {
+          'ember-source': null
+        }
       }
     },
     {
@@ -102,7 +113,9 @@ module.exports = {
         }
       },
       npm: {
-        'ember-source': null
+        devDependencies: {
+          'ember-source': null
+        }
       }
     }
   ]