Use travis' container-based infrastructure (#1314)

Use travis' container-based infrastructure (no sudo)

.travis.yml

@@ -1,14 +1,12 @@
 language: generic
-
+sudo: false
-branches:
+addons:
-  only:
+  apt:
-    - master
+    packages:
-
+    - luarocks
-sudo: required
-
 before_install:
-  - sudo apt-get update
+  - luarocks install --local luacheck
-  - sudo apt-get install -y luarocks
+script: 
-  - sudo luarocks install luacheck
+- $HOME/.luarocks/bin/luacheck --no-color ./mods
-
+notifications:
-script: luacheck --no-color ./mods
+  email: false