Add messageformat as dev dependency, req. node >= 6.0, add npm lockfile

Author: eemeli

package-lock.json

@@ -13,7 +13,13 @@
   "repository": "https://github.com/eemeli/gettext-to-messageformat",
   "author": "Eemeli Aro <[email protected]>",
   "license": "MIT",
+  "engines": {
+    "node": ">=6.0"
+  },
   "dependencies": {
     "gettext-parser": "^1.3.0"
+  },
+  "devDependencies": {
+    "messageformat": "^1.0.2"
   }
 }