Improve output from `rspec -v`.

Created by: myronmarston

Before this change, this confusing thing could happen:

$ gem install rspec -v 3.5.0 > /dev/null; rspec -v
3.5.1

The user installed RSpec 3.5.0 but rspec -v prints 3.5.1. This happened because 3.5.0 of the rspec gem depends on 3.5.x of all the sub-gems, and 3.5.1 is the latest rspec-core.

With this change, it now prints something like:

$ rspec -v
RSpec 3.5
  - rspec-core 3.5.1
  - rspec-expectations 3.5.0
  - rspec-mocks 3.5.0
  - rspec-rails not installed
  - rspec-support 3.5.0