Transform `async` functions to bluebird promises

It turns out that the assertion made in
https://github.com/matrix-org/matrix-react-sdk/pull/1213 about `async`
functions returning bluebird promises was only correct when babel used an
inline version of the `asyncToGenerator` helper; in react-sdk we are using
`babel-transform-runtime` which means that we use a separate
`babel-runtime/helpers/asyncToGenerator`, which returns a native (or core-js)
Promise.

This meant that we were still in the situation where some methods returned
native Promises, and some bluebird ones, which is exactly the situation I
wanted to resolve by switching to bluebird in the first place: in short,
unless/until we get rid of all code which assumes Promises have a `done` method
etc, we need to make sure that everything returns a bluebird promise.

(Aside: there was debate over whether in the long term we should be trying to
wean ourselves off bluebird promises by assuming all promises are native. The
conclusion was that the complexity hit involved in doing so outweighed any
benefit of a potential future migration away from bluebird).

.babelrc

@@ -1,4 +1,4 @@
 {
   "presets": ["react", "es2015", "es2016"],
-  "plugins": ["transform-class-properties", "transform-object-rest-spread", "transform-async-to-generator", "transform-runtime", "add-module-exports"]
+  "plugins": ["transform-class-properties", "transform-object-rest-spread", "transform-async-to-bluebird", "transform-runtime", "add-module-exports"]
 }

package.json

@@ -85,7 +85,7 @@
     "babel-eslint": "^6.1.2",
     "babel-loader": "^6.2.5",
     "babel-plugin-add-module-exports": "^0.2.1",
-    "babel-plugin-transform-async-to-generator": "^6.16.0",
+    "babel-plugin-transform-async-to-bluebird": "^1.1.1",
     "babel-plugin-transform-class-properties": "^6.16.0",
     "babel-plugin-transform-object-rest-spread": "^6.16.0",
     "babel-plugin-transform-runtime": "^6.15.0",