Enable colored Luacheck output on Travis CI

This also removes the deprecated `sudo: false` option, as Travis CI
is phasing out the container-based infrastructure.

.travis.yml

 language: generic
-sudo: false
 addons:
   apt:
     packages:
     - luarocks
 before_install:
   - luarocks install --local luacheck
-script: 
-- $HOME/.luarocks/bin/luacheck --no-color ./mods
+script:
+- $HOME/.luarocks/bin/luacheck ./mods
 notifications:
   email: false