trezorctl: update url to releases.json

[bithobbes]

No description provided.

[prusnak]

I think we can implement the following improvements:

a) use connect().features.bootloader_mode to check whether the device is in the bootloader mode first and throw an exception if not

b) use connect().features.major_version to determine whether we should put 1 or 2 in the JSON url

Would you like to try it out?

[prusnak]

Closing as unmerged. This needs to be handled in a more systematic way.

Let's deal with that in #269