mod_storage_sql: Spell out missing dependencies
Using util.dependencies appeared to cause problems with running tests in
Busted, so this also removes that and uses pcall directly.
For full information on our dependencies, version requirements, and
where to find them, see https://prosody.im/doc/depends
If you have luarocks available on your platform, install the following:
- luaexpat
- luasocket
- luafilesystem
- luasec