Use APT pinning to fix the Sensu version

This stops version bouncing (and possible related breakage) when the
node's specified Sensu version is lower than the latest version
available in the repository, and the node is upgraded via apt-get
(without pinning, Sensu will be upgraded by APT, then downgraded at
next Chef client run).
Merge pull request #172 from pdf/apt_pin_version

Use APT pinning to fix the Sensu version