Pass the browser version through Babel

This will allow to use ES2015 constructs in PEG.js code.

See #442.

Makefile

@@ -74,7 +74,10 @@ browser:
 	echo ' * Licensed under the MIT license.'                                          >> $(BROWSER_FILE_DEV)
 	echo ' */'                                                                         >> $(BROWSER_FILE_DEV)
 
-	$(BROWSERIFY) --standalone peg $(MAIN_FILE) >> $(BROWSER_FILE_DEV)
+	$(BROWSERIFY)                                                   \
+		--standalone peg                                              \
+		--transform [ babelify --presets [ es2015 ] --compact false ] \
+		$(MAIN_FILE) >> $(BROWSER_FILE_DEV)
 
 	$(UGLIFYJS)                 \
 	  --mangle                  \

package.json

@@ -48,6 +48,8 @@
     "test": "make lint && make spec"
   },
   "devDependencies": {
+    "babel-preset-es2015": "6.14.0",
+    "babelify": "7.3.0",
     "browserify": "13.1.0",
     "eslint": "2.13.1",
     "http-server": "0.9.0",