Add a script to check the version of babel cli

Because the package has changed so npm can't just auto-upgrade,
so this at least tells people how to fix it when the upgrade
breaks it for everybody.

babelcheck.js

@@ -0,0 +1,19 @@
+var exec = require('child_process').exec;
+
+// Makes sure the babel executable in the path is babel 6 (or greater), not
+// babel 5, which it is if you upgrade from an older version of react-sdk and
+// run 'npm install' since the package has changed to babel-cli, so 'babel'
+// remains installed and the executable in node_modules/.bin remains as babel
+// 5.
+
+exec("babel -V", function (error, stdout, stderr) {
+    if (parseInt(stdout.substr(0,1), 10) < 6) {
+        console.log("\033[31m\033[1m"+
+            '*****************************************\n'+
+            '* matrix-react-sdk has moved to babel 6 *\n'+
+            '* Please "rm -rf node_modules && npm i" *\n'+
+            '*****************************************\n'+
+        "\033[91m");
+        process.exit(1);
+    }
+});

package.json

@@ -14,8 +14,8 @@
   },
   "scripts": {
     "reskindex": "./reskindex.js -h header",
-    "build": "babel src -d lib --source-maps",
-    "start": "babel src -w -d lib --source-maps",
+    "build": "node babelcheck.js && babel src -d lib --source-maps",
+    "start": "node babelcheck.js && babel src -w -d lib --source-maps",
     "lint": "eslint src/",
     "lintall": "eslint src/ test/",
     "clean": "rimraf lib",
@@ -89,5 +89,8 @@
     "sinon": "^1.17.3",
     "source-map-loader": "^0.1.5",
     "webpack": "^1.12.14"
+  },
+  "optionalDependencies": {
+    "babel": "6.5.2"
   }
 }