Update the rockspec so that the Luarocks install works.

I just tried to add a hotkey to call `grid.adjustheight(x)` into my configuration and noticed that it didn't work. I reinstalled the latest via

~~~bash
luarocks install mjolnir.bg.grid
~~~
and then tested to make sure that worked
~~~bash
luarocks show mjolnir.bg.grid

mjolnir.bg.grid 0.1-3 - Mjolnir module for moving/resizing your windows along both virtual and horizontal grids.

Mjolnir module for moving/resizing your windows along both virtual and
horizontal grids.

License: 	MIT
Homepage: 	https://github.com/BrianGilbert/mjolnir.bg.grid
Installed in: 	/usr/local

Modules:
	mjolnir.bg.grid (/usr/local/share/lua/5.2/mjolnir/bg/grid.lua)

Depends on:
	mjolnir.fnutils
	mjolnir.application
	mjolnir.geometry
	mjolnir.alert
	mjolnir.screen
~~~

Looking at your repo and the git log, I *think* you merged the commit to fix the adjustheight method and forgot to update the rockspec. This fixes that so all you have to do is push it up to Luarocks.