Intall packages on install step

The `install` step runs before the `script` step, no reason not to just 
use it here.

Author: vzamanillo

.travis.yml

@@ -14,7 +14,7 @@ env:
   global:
     APM_TEST_PACKAGES: "language-lua"
 
-before_install:
+install:
   - sudo apt-get update
   - sudo apt-get install --assume-yes --quiet --no-install-suggests --no-install-recommends lua5.3 luajit