Migrate to container based travis-ci builds

.travis.yml

+sudo: false
+addons:
+  apt:
+    packages:
+    - libwxgtk3.0-dev
+    - libgtk2.0-dev
 before_install:
-    - sudo apt-get update -qq
-    - sudo apt-get install -y libwxgtk3.0-dev libgtk2.0-dev
+    - #sudo apt-get update -qq
+    - #sudo apt-get install -y libwxgtk3.0-dev libgtk2.0-dev
     - git show -s --format="wxT(\"<a href=\\\"http://github.com/audacity/audacity/commit/%H\\\">%h</a> of %cd\")"
     - git show -s --format="wxT(\"<a href=\\\"http://github.com/audacity/audacity/commit/%H\\\">%h</a> of %cd\")" > ./src/RevisionIdent.h
 language: