Update build toolchain to Babel 7.0

Rollup is useful for bundling several source files together, but for
single file builds we can just use Babel directly and avoid an extra
dependency.

Author: dgraham

.babelrc

@@ -0,0 +1,16 @@
+{
+  "env": {
+    "esm": {
+      "presets": [
+        ["@babel/env", {"modules": false}],
+        "@babel/flow"
+      ]
+    },
+    "umd": {
+      "plugins": [
+        "@babel/plugin-transform-modules-umd"
+      ],
+      "presets": ["@babel/env", "@babel/flow"]
+    }
+  }
+}