Fix generating standalone script in bundle context

`bundle exec rake standalone` in development mode would try to shell out
to `ruby --disable-gems` to detect 1.9 support, but that would fail
because Bundler tries to propagate itself through RUBYOPT and
`--disable-gems` obviously doesn't play along with it.

lib/hub/standalone.rb

@@ -76,7 +76,7 @@ preamble
 
     def ruby_shebang
       ruby = ruby_executable
-      `#{ruby_executable} --disable-gems -e0 2>/dev/null`
+      `RUBYOPT= #{ruby_executable} --disable-gems -e0 2>/dev/null`
       if $?.success?
         "#{ruby} -E UTF-8 --disable-gems"
       else