maint - increase required api version to 2.6.0

When implementing MCO-443 we added MCollective::Client.request_sequence and
made use of it in the connectors.  This commit completes this by bumping the
required mcollective api version to 2.6.0, the first version that will have
that new api.
Merge pull request #232 from richardc/connector_api_version

maint - increase required api version to 2.6.0