Create symlink to newly-installed firefox

Some test runners may modify $PATH while they run, and invoking
firefox can result in surprising errors if they expect
certain version of firefox instead of the installed default.

So, instead, we overwrite the default firefox symbolic link.

You can see the simulated result here:

https://travis-ci.org/BanzaiMan/travis_production_test/builds/40940104
Auto-merged master into ha-bug-symlink-firefox on deployment.
Move firefox addon to after_prepare

This allows use of `sudo`.

We also validate the value given from the user, and abort if
it doesn't match the expected pattern.
Merge branch 'ha-bug-symlink-firefox' of github.com:travis-ci/travis-build into ha-bug-symlink-firefox
Fix expected code in Firefox addon spec
Do not return prematurely when invalid firefox version is given
Strip user input for firefox addon version
Ensure that HHVM defined session.save_path is writable
Update url of firefox

http://releases.mozilla.org/pub/firefox/releases has moved http://download.cdn.mozilla.net/pub/firefox/releases permanently
Merge pull request #343 from PeterDaveHello/patch-1

Update url of firefox
Add Google and OpenDNS nameservers

There are reports of DNS lookups failing with some frequency,
and they cause build to fail.
(https://github.com/travis-ci/travis-ci/issues/2965)

This commit aims to address it by adding publicly available and
robust name servers from Google and OpenDNS.
Revert "Update url of firefox"

This reverts commit ad672db00c9fa5fd40f691c7b288dc349ceaebfc.
Spell check variable name
Auto-merged master into ha-feature-dns-fix on deployment.
Merge pull request #344 from travis-ci/ha-feature-dns-fix

Add Google and OpenDNS nameservers
replace set with sh.export
Merge pull request #339 from travis-ci/ha-bug-symlink-firefox

Create symlink to newly-installed firefox
Merge pull request #342 from scfc/fix-hhvm-var-lib-php5-permissions

Ensure that HHVM defined session.save_path is writable
Merge pull request #345 from MartinNowak/fixInit

replace set with sh.export
Drop '--jobs' from bundler args when running on Rubinius

This closes https://github.com/travis-ci/travis-ci/issues/2952
and provides a workaround for https://github.com/travis-ci/travis-ci/issues/2821