Implement `add_conflicts`. Solve #2383.

Usage:
```ruby
spec.add_conflicts 'mruby-gem-x', 'mruby-gem-y'
```

Format of `@conflicts` instance variable is array of conflicting mrbgem name.
I've tried `add_conflict` method that could take version requirements as rest arguments.
Though it was same as `add_dependency` so I didn't included it to this patch.