Restore default of remote search to `gem search`
Restore block behavior for RemoteFetcher#request

Before #283 was merged RemoteRequest#request would yield the request
object to a block for customization.  This behavior lacked tests so it
was accidentally removed.

Now the behavior has been restored and a test has been added.

Fixes #602
Fix warning
Force use of minitest 4.x when 5.x is present

Ruby trunk has not been updated to minitest 5.x yet so the tests have
warnings and some errors due to incompatibilities.

Now minitest 4.x is forced through both `rake` and `ruby -Ilib test/...`
Use testcase-defined Gem.ruby=

This is cleaner than setting the instance variables
Fix test failure when ruby is not yet installed

Other uses of ruby in a Makefile use ENV['RUBY'] which contains a
miniruby invocation when ruby is not yet installed (tests run during the
ruby build).
Ignore missing Kernel#gem for ruby tests
Update history for 2.0.6
Set release-candidate version
Set licenses
Improve performance of `gem outdated`

Added Gem::Specification::outdated_and_latest_version which enumerates
outdated local specifications and the latest version.  This removes
duplicate work between Gem::Specification::outdated and the
OutdatedCommand.

On 244 gems I see 154 seconds for RubyGems 2.0.x and 95 seconds for this
version.
Use full cipher name for the key cipher

This addresses the specific error for #606 but not the general problem
of running the tests on JRuby.
Encrypt test key with DES-CBC for JRuby

AES-256-CBC does not work for encrypting a key on JRuby, see jruby/jruby#919

Part of #606
Increase key sizes and avoid aliases for JRuby

Part of #606
Updated history for #606
Add util/create_encrypted_key.rb to Manifest
Extract extension_builder from build_extensions
Extract build_extension from build_extensions
Move extension building to Gem::Ext::Builder

The rest of the extension building functionality is already located
under ext/ so it makes sense to move the driver to this existing empty
class.

This is groundwork for fixing #607 in a maintainable way.
Move extension building tests