fix compatibility with mcollective > 2.4

The mcollective > 2.4 have a new method
MCollective::Util.str_to_bool,
if using mcollective-client with gem < 2.4
and using mcollective > 2.4 the discovery
works only for nodes and collectives.

And the agent discovery gives error
undefined method MCollective::Util.str_to_bool.

This change makes mcomaster compatible with
mcollective > 2.4, and drops compatibility with
mcollective < 2.4.

Update dependency list on INSTALL.md

We require that user have mcollective > 2.4
Merge pull request #20 from lmello/support_mcollective_2.4

fix compatibility with mcollective > 2.4