Updated README.md to use https to clone ruby-build
cache git clone directory

if RUBY_BUILD_CACHE_PATH is set, create and maintain a local bare git
repository in this location.  The name of the git repository is based
on the git URL, so it will be shared between branches.
Address some feedback from Jeremy

* Decrease likelihood of cache collisions
* Avoid file:// urls (and therefore the need to escape)
Extract `sanitize` helper
Default RBENV_VERSION to the globally-specified Ruby

Fixes `rbenv install` in a directory with a .ruby-version file that specifies ree
Pass $RUBY_CONFIGURE_OPTS to REE installer with -c
Build REE --without-tk on Darwin if X11 is missing

Fixes #267
Merge pull request #337 from mhuffnagle/UpdateReadme

Unable to clone ruby-build from behind firewall
Restore -O3 default when we build with clang
Merge pull request #351 from jeremy/restore-o3-cflags

Restore -O3 default when we build with clang
ruby-build 20130501
Avoid touching CFLAGS unless explicitly requested (#352)
Typo (#353)
Allow define RUBY_CONFIGURE_OPTS on rbx configure

The configure of rbx can take a RUBY_CONFIGURE_OPTS. You can by example
use the `--skip-prebuilt` OPTS
Merge pull request #354 from shingara/features/configure_on_rbx

Allow define RUBY_CONFIGURE_OPTS on rbx configure
Add a definition for Ruby 1.9.1-p430
Add a definition for Ruby 1.9.2-p0
added 1.9.3-p426 and 2.0.0-p195
Merge pull request #356 from hsbt/added-latest-releases

added 1.9.3-p426 and 2.0.0-p195
Replacing 1.9.3-p426 with 1.9.3-p429

1.9.3-p426 was deprecated by 1.9.3-p429 since it won't build on RHEL6
systems (and others?)