With new Firefox 57 and WebExtensions, the new addon uses Firefox'
Native Messaging mechanism to communicate with local programs (here
the main enpass program). This commit activates EnpassNMHost, a binary
that bootstraps communication between the two.
The file was generated with the update script that is part
of the nix expressions for enpass.
Also, it seems that 5.4 has some issues with dropbox sync,
this was the original rationale to look for a newer version.
