Add rbconfig configuration for Ruby 2.0.0

These known/unknown platform pairs are required for tests to run
under Ruby 2.0.0
Avoid Bundler poluting cross-compilation

Since we started to use Bundler, it modified RUBYOPT environment
variable which affected cross compilation scenarios.

Unset RUBYOPT variable if Bundler is defined when running Cucumber.
Update versions used for bootstrapping
Update default version used for cross-compilation

1.8.7-p371 has been quite stable ;-)
Use RUBY_PLATFORM format and not Gem-like platform

Do this to avoid issues between rbconfig platform matching and
Gem compilation