Make compatible with ruby 1.8.7

Some changes to make it work with ruby 1.8.7.

I changed some examples in the README to avoid errors using ruby 1.8.7 due to the lack of block scope variables.

# Test results

    $ ruby --version
    ruby 1.8.7 (2012-10-12 patchlevel 371) [x86_64-linux]
    $ rake spec
    ~/.rvm/rubies/ruby-1.8.7-p371/bin/ruby -S rspec ./spec/jvmargs/args_spec.rb ./spec/jvmargs/directive_spec.rb ./spec/jvmargs/nonstandard_spec.rb ./spec/jvmargs/standard_spec.rb ./spec/jvmargs/unstable_spec.rb ./spec/jvmargs/util_spec.rb
    .................................
    
    Finished in 0.09036 seconds
    33 examples, 0 failures