Fixed crash if a .gemspec has a string for require_paths variable.
Fixed text from require_path to require_paths.  Ensured Array() on require_paths.
From attr_writer :require_paths to defined with require_paths= which sets with Array.  Using raw_require_paths as @zzak did.
In .to_ruby changed @require_paths to raw_require_paths
Merge branch 'master' of https://github.com/danielpclark/rubygems
Updated attr_writer :require_paths to def require_paths= which sets @require_paths with an Array
Updated attr_writer :require_paths to method assigning an Array.
A lockfile dependency may not exist in all sets

When looking up a lockfile dependency we did not account for a set not
having a matching specification at all.  Now we ignore sets without the
specification.

Part of #897
Add #897 to History
Document single-digit behavior of ~>

Also, add explicit tests of the zero-extension behavior so nobody breaks
it by accident.

Fixes #896
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
Add #913 to History
Remove circular requires

Now rubygems/request_set and rubygems/resolver must be required after
rubygems (which you already loaded anyways).

Fixes #908
Merge pull request #909 from danielpclark/master

Ensure Array assignment on require_paths
Add #904 to History

Add #909 to History
Use Native File::PATH_SEPARATOR and remove $ before gem env

Address @luislavena's comment: https://github.com/rubygems/rubygems/pull/913#discussion_r12563149
Merge pull request #915 from parkr/patch-1

Use Native File::PATH_SEPARATOR and remove $ before gem env
Add History for #915
Support /prerelease_specs URLs in Gem::Server

Gem::Server did not support prerelease gems at all.  This adds support
for listing them but not installing them (fetching the specs).

Part of #857