Add TravisCI integration

This PR contains a working `.travis.yml` that builds ack2.

However, it is currently configured to build agains _my_ repo. To make travis be notified when there is a push to yours, you must to the following.

1. Go to https://travis-ci.org and sign in with your GitHub credentials
2. Flip the switch on the repository
3. Change the URLs of the build status badge in README.md to show your repo's build job

There is one issue though: Some tests can't be executed and give the following error instead:

    skipped: You need to install IO::Pty to run this test

I tried installing `IO:Pty` manually with a `cpanm IO:Pty` but that didn't work: https://travis-ci.org/lenniboy/ack2/builds/7057563

I'm not a Perl developer (just a ack and travis lover). If you could help me out how to install the dependency properly, I'd gladly set it up.