Add coverity static analysis to Travis CI config

Add coverity static analysis to Travis CI config

Enable coverity analysis for the release/2.4 branch.

We can only do a limited number of coverity scans per week with our FOSS
account, but since we only occasionally push commits, that should work out
fine.  But this limit is the reason we don't use the standard travis addon,
because that would cause the coverity script to run on all of our matrix
builds.  That would cause us to reach our limit faster, and waste travis'
resources.

Since our FOSS coverity account doesn't handle multiple branches very well,
we have to pick one branch to run coverity on.  I think it's best to use
the most recent stable branch for that (i.e. for now, release/2.4).
Though for ease of maintenance, it's probably best to apply the patch to
both master and release/2.4.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Antonio Quartulli <a@unstable.cc>
Message-Id: <1502207741-31750-1-git-send-email-steffan.karger@fox-it.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15176.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit 4a05f15c9aafe314ae4d3642813ebf234c09276e)