ruby-build 20120524

• Add JRuby 1.6.7.2 and 1.7.0-preview1
• Removed the definition for JRuby 1.7.0-dev (in general we do not
  like to remove definitions, but the JRuby team has deleted the
  1.7.0-dev package from their servers -- caveat emptor)
• Added support for specifying the build location with the
  RUBY_BUILD_BUILD_PATH environment variable
• Add a -k/--keep flag to ruby-build and rbenv install for
  keeping the source code around after installation
• Update the README to emphasize installation as an rbenv plugin