Remove obsolete babel dependency (#74)

`babel` no longer does anything since Babel v6.

Author: goto-bus-stop

package.json

@@ -33,7 +33,6 @@
   },
   "homepage": "https://github.com/tus/tus-js-client",
   "devDependencies": {
-    "babel": "^6.5.2",
     "babel-cli": "^6.6.5",
     "babel-eslint": "^4.1.5",
     "babel-preset-es2015": "^6.1.18",