Renamed letsencrypt.js to greenlock.js;

Updated package.json to reference only greenlock.js.

package.json

@@ -4,8 +4,8 @@
   "description": "CLI for node-greenlock modeled after the official client",
   "main": "index.js",
   "bin": {
-    "letsencrypt": "bin/letsencrypt.js",
-    "letsencrypt-node": "bin/letsencrypt.js",
+    "letsencrypt": "bin/greenlock.js",
+    "letsencrypt-node": "bin/greenlock.js",
     "greenlock": "bin/greenlock.js",
     "greenlock-node": "bin/greenlock.js"
   },