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

@@ -1,5 +1,4 @@
 language: generic
-sudo: false
 addons:
   apt:
     packages:
@@ -7,6 +6,6 @@ addons:
 before_install:
   - luarocks install --local luacheck
 script:
-- $HOME/.luarocks/bin/luacheck --no-color ./mods
+- $HOME/.luarocks/bin/luacheck ./mods
 notifications:
   email: false