get_fly method should automatically download same version as server

[vinaykonanki]

Right now, in fly.py (get_fly) we are just checking if the fly binary exists in DEFAULT_FLY_BIN, but apparently, we are not checking if the version of the fly CLI is the same as the server.

When we upgrade the server to the latest version, all of a sudden our client program fails (It throws warning that fly is out of sync). We should enhance the get_fly method by not just checking if the fly binary exists but also if it is same as server, if not we need to run the "fly sync" command.

Error: WARNING:

fly version (5.0.1) is out of sync with the target (5.2.0). to sync up, run the following:

    fly -t default sync