travis: disable caching, remove apt-install

.travis.yml

@@ -2,12 +2,7 @@ dist: trusty
 language: ruby
 sudo: true # need this for libgirepository1.0-dev
 # sudo: false # taking advantage of container-based infrastructure
-cache: bundler # cache bundled gems from build to build
+# cache: bundler # cache bundled gems from build to build
-
-addons:
-  apt:
-    packages:
-      - libgirepository1.0-dev # for gobject-introspection
 
 rvm:
   - 2.2