Improve path searching during auto activation. Fixes #227
Skip dev deps checking on unistall by default. Fixes #329
Ignore failures on 1.9.2 and ruby-head for now
Clarify comment about how Gem.try_activate works
Show prerelease in Dependency#inspect
Disable background rdoc generation by default

Right now, it's terribly annoying because your CPU unexpectedly and the
background process continues to spew messages into the terminal with no
way to stop them.
Fix typo bug
Introduce Gem::AvailableSet

AvailableSet abstracts tracking and picking which gems to install from
which sources.

Previously, the [[spec, source], ...] array had an implicit order that
was required in order for RubyGems to function properly. This is now
properly documented and maintained by AvailableSet.
Style fix
Fix warning generated by "install -i"

rubygems/rdoc uses the rdoc gem, which uses the json gem. If there are
multiple json gems installed, it because unresolved (because it's
ambigious) and a warning is emitted when we do Specification.reset. This
fixes that warning by forcing a json gem to be activated.
Update Manifest
avoid endless loops for custom_shebang
cleaning installer/shebang
Merge pull request #330 from mpapis/master

custom_shebang should not be used on binaries used in it - prevent loop
Merge pull request #384 from mpapis/clean_330

Clean up for #330
No reason darwin has a specific treatment.

Now, a version like unknown-freebsd or x86-openbsd will work.
Add some tests for the platform fix.
! Removed broken YAML gemspec support in `gem build`
Merge pull request #386 from Absolight/fixup-platforms

No reason darwin has a specific treatment.
Don't modify entries in $LOAD_PATH directly

It is not allowed in ruby 2.0.0 since r37481.

See also:
  https://bugs.ruby-lang.org/issues/7158
  https://bugs.ruby-lang.org/projects/ruby-trunk/repository/revisions/37481