maint - the official gem is called mcollective-client

When I added the initial travis testing in 779969 I feel into the not uncommon
trap of thinking the official gem would be called 'mcollective' and not
'mcollective-client'.

Here we tweak the Gemfile to test against that instead
maint - add mcollective 2.4.x to travis matrix
Merge pull request #14 from richardc/travis_2_4

Add MCollective 2.4.x to the testing matrix