Use travis' container-based infrastructure (#1314)

Use travis' container-based infrastructure (no sudo)

.travis.yml

 language: generic
-
-branches:
-  only:
-    - master
-
-sudo: required
-
+sudo: false
+addons:
+  apt:
+    packages:
+    - luarocks
 before_install:
-  - sudo apt-get update
-  - sudo apt-get install -y luarocks
-  - sudo luarocks install luacheck
-
-script: luacheck --no-color ./mods
+  - luarocks install --local luacheck
+script: 
+- $HOME/.luarocks/bin/luacheck --no-color ./mods
+notifications:
+  email: false