Give more error information on dependency fails

When rubygems cannot find your installed gems it will raise an error. It includes the name of the gem it was looking for and a count of the available gems in the available path, but it does not give any additional debugging information. 

This PR adds the path locations of where gems are stored so the developer could manually check to see if the gem exists in one of them. If there is not an issue with GEM_PATH then perhaps a quick check of `$ gem env` will at least prompt users to dig in more. 

When I first started developing in Ruby I had many gem problems, most of them were solved after posting to ruby-forum.com and them asking for my `gem env`. By prompting the user to look for that information proactively we can decrease the friction of debugging these errors.

Old error:

```
/Users/schneems/.rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/dependency.rb:300:in `to_specs': Could not find 'bundler' (>= 0) among 8 total gem(s) (Gem::LoadError)
	from /Users/schneems/.rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/dependency.rb:311:in `to_spec'
	from /Users/schneems/.rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_gem.rb:53:in `gem'
	from /tmp/bin/bundle:18:in `<main>'

```


New error:

```
/Users/schneems/.rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/dependency.rb:300:in `to_specs': Could not find 'bundler' (>= 0) among 8 total gem(s) (Gem::LoadError)
Checked in 'GEM_PATH=foo:bar', execute `$ gem env` for more information
	from /Users/schneems/.rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/dependency.rb:311:in `to_spec'
	from /Users/schneems/.rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_gem.rb:53:in `gem'
	from /tmp/bin/bundle:18:in `<main>'
```
Merge pull request #913 from schneems/schneems/gem-dependency-error

Give more error information on dependency fails