Add support for Travis CI (http://travis-ci.org).

Simple CI for branches of JUnit forks. Now everybody can easily use CI for own branches. Pull requests are automatically build by the CI and the build result is visible on Github.
Merge pull request #776 from stefanbirkner/travis

Add support for Travis CI (http://travis-ci.org).
Merge branch 'master' of https://github.com/junit-team/junit