Pin chef version to prevent bunny conflicts

This pull request adds a requirement on a version of the chef gem which
is known to work with our other dependencies. Without this runtime
dependency, a newer chef gem can require conflicting libraries (version
of the bunny gem for e.g.) that prevent Ohai plugins from loading
correctly.

The upshot of all this, is that the chef_environment ohai plugin wasn't
loading and all our servers defaulted to running in the development
environment.

This PR fixes that situation and should hopefully ensure that the
chef_environment ohai_plugin is correctly loaded. An example ohai plugin
almost identical to what we use is in the test fixtures.