Use Chrome PPA, update chromedriver, firefox

* No need to download our specific version
* Use officially supplied FF 45

playbooks/roles/browsers/defaults/main.yml

 browser_deb_pkgs:
-    - xvfb
     - dbus-x11
+    - firefox=45.0.2+build1-0ubuntu1
+    - google-chrome-stable
+    - libcurl3
     - libgconf2-4
-    - libxss1
     - libnss3-1d
-    - libcurl3
+    - libxss1
     - xdg-utils
-    - gdebi
-
-# Debian packages we host in S3 to ensure correct browser version
-# Both Chrome and FireFox update their apt repos with the latest version,
-# which often causes spurious acceptance test failures.
-browser_s3_deb_pkgs:
-    - { name: "google-chrome-stable_30.0.1599.114-1_amd64.deb", url: "https://s3.amazonaws.com/vagrant.testeng.edx.org/google-chrome-stable_30.0.1599.114-1_amd64.deb" }
-    - { name: "firefox-mozilla-build_42.0-0ubuntu1_amd64.deb", url: "https://s3.amazonaws.com/vagrant.testeng.edx.org/firefox-mozilla-build_42.0-0ubuntu1_amd64.deb" }
+    - xvfb
 
 # Chrome and ChromeDriver
-chromedriver_version: 2.6
+chrome_repo: "deb [arch=amd64] http://dl.google.com/linux/chrome/deb/ stable main"
+chromedriver_version: 2.25
 chromedriver_url: "http://chromedriver.storage.googleapis.com/{{ chromedriver_version }}/chromedriver_linux64.zip"
 
 # PhantomJS

playbooks/roles/browsers/tasks/main.yml

 # Install browsers required to run the JavaScript
 # and acceptance test suite locally without a display
 ---
+- name: install Google public key for apt
+  apt_key:
+    url: "https://dl-ssl.google.com/linux/linux_signing_key.pub"
+    state: present
+
+- name: add Google Chrome repo to the sources list
+  apt_repository:
+    repo: "{{ chrome_repo }}"
+    state: present
+
 - name: install system packages
   apt: pkg={{','.join(browser_deb_pkgs)}}
        state=present update_cache=yes
 
-- name: download browser debian packages from S3
-  get_url: dest="/tmp/{{ item.name }}" url="{{ item.url }}"
-  register: download_deb
-  with_items: "{{ browser_s3_deb_pkgs }}"
-
-- name: install browser debian packages
-  shell: gdebi -nq /tmp/{{ item.name }}
-  when: download_deb.changed
-  with_items: "{{ browser_s3_deb_pkgs }}"
-
-# Because the source location has been deprecated, we need to
-# ensure it does not interfere with subsequent apt commands
-- name: remove google chrome debian source list
-  file:
-    path: /etc/apt/sources.list.d/google-chrome.list
-    state: absent
-
 - name: download ChromeDriver
   get_url:
     url={{ chromedriver_url }}