Add travis ci check

This pull request provides initial continuous integration testing based on the same Travis-ci tool used other zeromq projects.

Buildbot is currently only working with Python2 only appropriate builders are enbaled in the Travis configuration file.

IMPORTANT:
There are a few things you will need to do during the merge step to properly integrate this change set:

1. Enable the zeromq-buildbot project within zeromq's Travis-ci.org account by selecting 'Accounts' from the profile drop-down at top right. In the Repositories tab you will need to enable zeromq/zeromq-buildbot repo. This will set up a service hook within the Github zeromq/zeromq-buildbot project so that a commit will trigger the Travis-ci build.

2. Modify the build status badge link in the README.md file to point at the correct url. It should be this:

<pre>
[![Build Status](https://travis-ci.org/zeromq/zeromq-buildbot.png?branch=master)](https://travis-ci.org/zeromq/zeromq-buildbot)
</pre>