Set TRAVIS_SECURE_ENV_VARS to 'true' when secure var is set on Settings tab

Fixes https://github.com/travis-ci/travis-ci/issues/2820
Change firefox download location

releases.mozilla.org is CDN'd, while ftp.mozilla.org is not.
This should result in much faster downloads when the pre-installed
firefox is not sufficient.
Merge branch 'master' into ha-bug-gh-2820
Merge pull request #311 from travis-ci/ha-bug-gh-2820

Set TRAVIS_SECURE_ENV_VARS to 'true' when secure var is set on Settings tab