New structure for getting bootstrapped:

* mvn package will just build enough to execute JRuby. This seems
  more in line with package phase, since phases after this involve
  integration testing.
* Move gem installations to integration-test phase. Ruby tests
  seem more like integration testing, since they're almost all
  hitting large parts of JRuby rather than any single unit. This
  also solves issues with installing gems that depend on a fully
  functional package, like jruby-launcher.
* ant test-extended runs everything up through verify stage, which
  includes integration-test gem installs. It then proceeds to
  run rake test:extended against the fully-bootstrapped JRuby.
* rake test:extended runs unit tests (via -Ptest) and all Ruby
  unit tests, which for our purposes are integration tests.

So the workflow goes like this:

* After a clean checkout of JRuby, or if you want to ensure you
  are properly bootstrapped (with updated gems, for example), you
  would run "mvn verify".
* From that point, all JRuby commands should work, including gem,
  rake, rspec, and so on.
* Subsequent updates to jruby.jar can be achieved with just "mvn
  package" or "mvn", which is reasonably quick.
* Ruby tests can be run via rake, rspec, ant, or mspec.
Remove unused jruby-launcher gem.
Specify max memory for unit test runs.
Increase fork count and do not reuse to give unit tests isolation.
Add BUILDING file to help newcomers get bootstrapped.
Minor tweaks to BUILDING
Build status should show only the master

…until we branch 1.7
Merge remote-tracking branch 'github/master'
[skip ci] Build instructions are now obsolete