Update get_rocketchat_current_version.

[santalex]

Compatible with getting version in rocketchat.service from online documents.

Docs says: Environment=MONGO_URL=mongodb://localhost:27017/rocketchat?replicaSet=rs01 MONGO_OPLOG_URL=mongodb://localhost:27017/local?replicaSet=rs01 ROOT_URL=http://localhost:3000/ PORT=3000 and this script says: Environment=PORT=3000

[CLAassistant]

All committers have signed the CLA.

[santalex]

#42