build: Target luacheck HEAD.

https://github.com/mpeterv/luacheck/pull/81#issuecomment-261099606

> If you really want to use bleeding-edge version you should get the
> rockspec from master branch, not a fixed commit ...
> The correct way to install from a specific commit is cloning that
> commit and running "luarocks make" from project directory. The reason
> is that running "install" or "build" on an scm rockspec fetches
> sources from master but uses build description from the rockspec
> itself, which may be outdated.

test/functional/shada/variables_spec.lua

@@ -33,7 +33,7 @@ describe('ShaDa support code', function()
       else
         meths.set_var(varname, varval)
       end
-      -- Exit during `reset` is not a regular exit: it does not write shada 
+      -- Exit during `reset` is not a regular exit: it does not write shada
       -- automatically
       nvim_command('qall')
       reset()