Skip deployment from Travis CI for pull requests from forks

Pull requests from forks do not have TRAVIS_SECURE_ENV_VARS, and
therefore cannot be deployed to our CloudFoundry organization.

Prior to this change, Travis CI would attempt to deploy pull requests
from forks, resulting in an error and build failure.

Now, the value of TRAVIS_SECURE_ENV_VARS is taken into more careful
consideration (along with other build metadata) to ensure we attempt
a deploy only when it is actually possible. Otherwise, the deploy task's
onlyIf {} closure is configured to return 'false', thus causing the
deploy task to report "SKIPPED".