Add Travis-CI configuration to ensure the Gradle-based build doesn't break

Although the Gradle-based build works pretty well, and is very useful in
the repackaging of Bouncy Castle into Spongy Castle, it's not necessarily
used by all Bouncy Castle developers, and consequently it gets broken from
time to time, as new tests are added that don't work out-of-the-box.
Continuous Integration testing (as provided for free by Travis) will help
avoid this gradual corruption of the build, by letting devs know when
problems arise.

Bouncy Castle has some quite difficult-to-support build profiles (J2ME,
Java 1.1, 1.2, 1.3, etc) but this configuration just runs `gradle check`
against OpenJDK 1.7 - as a bare minimum, the test suite should pass on
that platform :)

Testing against OpenJDK also means we're not attempting to get the JCE
signed-provider stuff working, which is actually for the best, as uploading
the keystore that holds the signing key to the public repo is *not* on the
list of things you're allowed to do.

Note that after this configuration is merged, there is still a short set-up
process to perform through the Travis admin interface - see these links:

http://docs.travis-ci.com/user/getting-started/
http://docs.travis-ci.com/user/github-oauth-scopes/


Explaining some of the config options
=====================================

Setting TERM=dumb causes Gradle to use plain console output:

http://stackoverflow.com/a/17942820/438886


The '--info' flag hopefully will stop Travis builds from
dying from the no-output-in-10-minutes problem, but weirdly is
also the only thing that makes spongycastle tests pass...

https://twitter.com/rtyley/status/425373078084784128
https://travis-ci.org/rtyley/spongycastle/builds/17299026