support provider with path

example:

```ruby
  describe 'Gem' do
    let(:gem) { "/usr/local/rbenv/shims/gem" }

    %w(pry
       awesome_print
       bundler
       hub
       active_support).each do |p|
      describe package(p) do
        it { should be_installed.by gem }
      end
    end
  end
```