consider gem version to be 0.0.0 unless specified
consider empty version requirements as >= 0.0.0
Merge pull request #1242 from schmurfy/gem_version

consider gem version to be 0.0.0 unless specified
fix indentation
Rename parameter names for mrb_yield
Merge pull request #1248 from ktaobo/rename-yield-param

Rename parameter names for mrb_yield