Export ES6 from js-sdk

This is a somewhat experimental PR to make the build process target
ES6 rather than ES5 given ES6 is now widely supported.

#803

.babelrc

@@ -1,13 +1,13 @@
 {
-    "presets": ["es2015"],
-    "plugins": [
-        // this transforms async functions into generator functions, which
-        // are then made to use the regenerator module by babel's
-        // transform-regnerator plugin (which is enabled by es2015).
-        "transform-async-to-bluebird",
-
-        // This makes sure that the regenerator runtime is available to
-        // the transpiled code.
-        "transform-runtime",
+    "presets": [
+        [
+            "@babel/preset-env",
+            {
+                "targets": {
+                    // This targets ES6 (can't see a way to do this directly)
+                    "node": "6.5",
+                },
+            },
+        ]
     ],
 }