Update gem_package_task.rb

Check for RUBY_VERSION >= "2" is invalid, some earlier versions (e.g. 1.9) of ruby cannot use
rubygems/builder.
Avoid this error when using ruby 1.9 or jruby versions that act as 1.9:
    no such file to load -- rubygems/builder
This problem will appear as soon as the user's rake file does:
    require 'bones'
Problem appears in Jruby 1.7.5 using bones version 3.8.1