Update MagLev branch from master-1.9 to master

Leaves rubies >= 2.0 vulnerable to CVE-2013-6393

Ensures that color is off if the stderr stream is not a terminal

Require Java 7 for jruby-9000-dev.

Add support for rbx-2.2.5

Add JRuby 9000 snapshot.

This should plug the vulnerability to CVE-2013-6393 (and fix #504)
that can still occur in certain systems: If the ruby build process
couldn't find a libyaml that worked, it would build its own vendored
libyaml, which was 0.1.4 (and is vulnerable).

Instead, specify that the build always should install the latest
libyaml & build against that.

Actually resolves https://github.com/sstephenson/ruby-build/issues/499.

Closes https://github.com/sstephenson/ruby-build/issues/499.

Add MAKE_INSTALL_OPTS for packages

Closes #483

install(1) defaults to file mode 0755 which is inappropriate for Ruby
definitions

Also fix the ordering of make install arguments

Allows for passing environment variables specifically to the "make install"
step of package building.

This can be helpful for providing a different DESTDIR= to a ruby install from
the --prefix that is used

GNU sed compatibility

GNU sed version 4.1.5 on CentOS 5.8 does not support the -E flag:
$ sed -E
sed: invalid option -- E

Believe it or not, this was causing the following error:

    command not found-build: line 370: build_package_jruby

Closes #368

Benefits:
- makes it easier to stub `rbenv-*` subcommands in tests
- speeds up subcommands because it skips the main `rbenv` executable

Caveats:
- users are no longer able to call `bin/rbenv-install` or
  `rbenv-uninstall` directly