Juju trunk now is compatible with Mongo 2.6

[Ian Booth]

A little while back we started work as a background task to port Juju to work
with Mongo 2.6. After a final push from Andrew to get everything finalised, the
current Juju trunk now has Mongo 2.6 support finished/working.

Note that Mongo 2.4.x (as shipped in cloud archive and/or trusty) is still the
officially supported version for Juju.

The reason for this work was due to the fact that Mongo 2.6 offers various fixes
and improvements over 2.4; some of these fixes are for issues Juju encounters
but which will not be fixed for 2.4
eg https://jira.mongodb.org/browse/SERVER-11807
Also as time goes on, we need to have a viable pathway to move to Mongo 2.6 (and
later versions) since that's where most of the development focus will be. Plus
Mongo 2.6 is already in proposed for Utopic, so we now have the option to
package juju-mongodb with a consistent version of Mongo as ships with the distro.

What we will do now is set up regular test runs using Mongo 2.6 on CI so we'll
have data available about that combination's reliability etc.