Fix duplicate gemspec dependancy

RubyGems version ~> 2 breaks with

bundle_cache did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  duplicate dependency on bundler (~> 1.3, development), (>= 0) use:
    add_runtime_dependency 'bundler', '~> 1.3', '>= 0'