The websocket module comes with Prosody v0.10 directly, however the prosody-0.10 package on the Debian repo does not have a dependency on lua-bitop, which is required for mod_websocket.
We will probably take care of this closer to the 0.10.0 release.
Lua 5.2 comes with a bit lib built in.
Lua 5.3 removes this and adds bitwise operators.
I've started updating the package files, including a lua-bitop dependency.
Fixed in https://hg.prosody.im/debian/rev/b037cd8968b8