Mesos is now hosted in the Apache incubator. Please check out the source code from Apache's SVN:
svn checkout https://svn.apache.org/repos/asf/incubator/mesos/trunk
or its Git mirror:
git clone git://github.com/apache/mesos.git
For issue tracking, please use the JIRA issue tracker instead of the GitHub one.
Thanks!
-- The Mesos developers