Travis CI: Use the default firefox from ubuntu trusty

The alternative apt repository is no longer needed, which was used because the firefox version on trusty was very old, but this is no longer the case.

.travis/install_browsers.sh

@@ -1,7 +1,6 @@
 #!/usr/bin/env bash
 set -ex
 lsb_release -a
-sudo add-apt-repository -y ppa:ubuntu-mozilla-security/ppa
 sudo apt-get -qq update
 sudo apt-get install -y xvfb openbox chromium-browser firefox
 "$FIREFOX_BIN" --version