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
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
Merge pull request #346 from travis-ci/ha-bug-rbx-bundler-args

Drop '--jobs' from bundler args when running on Rubinius
untaint s3 cache urls since they are output to the script
Redo https://github.com/travis-ci/travis-build/pull/298

While the corresponding cookbooks change should be deployed,
we continue to receive reports of
https://github.com/travis-ci/travis-ci/issues/2514.

The PR bit to travis-build was missed during the sexp refactoring,
so we redo it with this commit.
Merge pull request #347 from travis-ci/sf-untaint-s3-options

Fix: untaint s3 cache urls since they are output to the script
Update sudo-less build message [skip ci]
Tweak sudo-less job message
Tweak sudo-less job message

Add a newline before the start of the message so it stands out.
Also, remove the newline in the middle of the message, because
the `:ansi` option does not span multiple lines.