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
Merge pull request #339 from travis-ci/ha-bug-symlink-firefox

Create symlink to newly-installed firefox