Improve child dependency conflict message

Hey folks,

This pull request fixes the issue #1128, I originally wrote it with my resolver pull request but now I found out it doesn't depend on any of the resolver code at all.

```ruby
source 'https://rubygems.org'

gem 'aruba', '0.3.6'
gem 'rspec-mocks', '2.6.0.rc2'
```

now properly gives,
```
Bundler could not find compatible versions for gem "rspec-mocks":
  In Gemfile:
aruba (= 0.3.6) ruby depends on
  rspec (>= 2.5.0) ruby depends on
    rspec-mocks (~> 2.5.0) ruby

    rspec-mocks (2.6.0.rc2)
```