Support babel v7 (#78)

We finally start supporting babel v7. As expected, it took more effort than just changing the number in package.json.

Among babel I updated other dependencies (apart from lint-staged, because in their changelog I saw they require Node >8.6, which I decided to fix within other PR).

Initial request: #30 

### Todo

- [x] Use new interface of @babel/cli (now we copy unwanted files)
- [x] Make sure tests for built version are passes

Author: ezhlobo

.babelrc

@@ -1,4 +1,14 @@
 {
-  "presets": ["forbeslindesay"],
-  "plugins": ["transform-flow-strip-types"]
+  "presets": [
+    ["@babel/preset-env", {
+      "targets": {
+        "node": "6",
+      },
+    }],
+    "@babel/preset-flow",
+  ],
+
+  "plugins": [
+    "@babel/plugin-proposal-class-properties",
+  ],
 }