fix gemspec

Fix warning

```
fluent-plugin-mail at /home/game/src/github.com/u-ichi/fluent-plugin-mail 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 fluentd (>= 0), (>= 0) use:
    add_runtime_dependency 'fluentd', '>= 0', '>= 0'
```