Add MAKE_INSTALL_OPTS for packages

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
Add tests for RUBY_MAKE_INSTALL_OPTS=
Add mention of RUBY_MAKE_INSTALL_OPTS in README
Add MAKE_INSTALL_OPTS configuration option
Add test for MAKE_INSTALL_OPTS

Also fix the ordering of make install arguments
Fix install file mode for files in share/

install(1) defaults to file mode 0755 which is inappropriate for Ruby
definitions
Test that files in `share/` get installed w/ mode 644
Merge branch 'install-mode'

Closes #483
Merge pull request #493 from eric/add-make-install-opts-variables

Add MAKE_INSTALL_OPTS for packages
Add Rubinius 2.2.4
Update libyaml to version 0.1.5

Closes https://github.com/sstephenson/ruby-build/issues/499.
ruby-build 20140204
Merge remote-tracking branch 'upstream/master'