.travis.yml: Disable IRC build status updates from forks

We want the travis build bot to post notifications on IRC only for the
master qemu repository and not the various forks/branches of
others. Currently there is no direct option to restrict the updates to
one repository. This is being worked upon by the developers and
tracked in https://github.com/travis-ci/travis-ci/issues/1094.

Until such time, we can use the workaround as posted in
ref. https://github.com/facebook/flow/pull/1822

.

This basically creates an ecrypted string which decrypts to qemu IRC
channel only on "qemu/qemu" repo and not on the forks. This enables
the build bot to notify the IRC only for the main repo.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>