Fixes error about 'private method XYZ called for Restforce:Configuration

This addresses the open issue #98 -- I was only able to repro this problem on Ruby 1.9.2 -- not on Ruby 2.0.0

When I changed the spec to use eval() instead of send(), I got what @bramswenson anticipated (log below) -- removing the private section of the Option class resolved the spec failure

```
  1) Restforce#configure allows authentication_callback to be set
     Failure/Error: eval "config.#{attr}= 'foobar'"
     NoMethodError:
       private method `authentication_callback=' called for #<Restforce::Configuration:0x0000000438e650>
     # ./spec/unit/config_spec.rb:61:in `eval'
     # ./spec/unit/config_spec.rb:61:in `eval'
     # ./spec/unit/config_spec.rb:61:in `block (5 levels) in <top (required)>'
     # ./lib/restforce/config.rb:26:in `configure'
     # ./spec/unit/config_spec.rb:60:in `block (4 levels) in <top (required)>'

  2) Restforce#configure allows mashify to be set
     Failure/Error: eval "config.#{attr}= 'foobar'"
     NoMethodError:
       private method `mashify=' called for #<Restforce::Configuration:0x0000000438b360>
     # ./spec/unit/config_spec.rb:61:in `eval'
     # ./spec/unit/config_spec.rb:61:in `eval'
     # ./spec/unit/config_spec.rb:61:in `block (5 levels) in <top (required)>'
     # ./lib/restforce/config.rb:26:in `configure'
     # ./spec/unit/config_spec.rb:60:in `block (4 levels) in <top (required)>'
```