add java as a valid RUBY_PLATFORM

[kaisecheng]

jruby users are not able to use env variable to setup proxy with basic authentication because the RUBY_PLATFORM does not allow java.
The commit includes java as a valid platform.

expected behaviour

1. export JRUBY_OPTS='-J-Dhttps.proxyHost=user:pw@host -J-Dhttps.proxyPort=3128 -J-Dhttp.proxyHost=user:pw@host -J-Dhttp.proxyPort=3128'
2. ruby -e "require 'net/http'; uri = URI('https://www.google.com'); Net::HTTP.get(uri);"
3. no error is thrown

Fixed: #68

[hsbt]

@kaisecheng Thanks, But this condition has been removed at #66