Fixes #23314 - Add Docker to virtual facts

Adds support to the virtual facts to detect if
Facter is running inside a Docker container.

It uses the /proc/1/environ file and checks for the
string:

docker=true

See http://www.freedesktop.org/wiki/Software/systemd/ContainerInterface/
for more details of this.

See related PR in Docker:

https://github.com/dotcloud/docker/pull/2939
Added RSpec Expectations to spec helper

Without this the RSpec tests failed to run with the error:

`alias_method': undefined method `should' for module
`RSpec::Matchers::BlockAliases' (NameError)

This was Ruby 1.9.3