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

Same approach taken in rbenv: sstephenson/rbenv@baf7656d

Add Rubinius 2.2.3

This reverts commit 36b5f33e.

See https://twitter.com/hsbt/status/415887230318567424.

Definitions of Ruby 2.1

Added definitions of Heroku's backports.

Closes #476