v4.1.0

Full Changelog

Merged pull requests:

• Implement MarshalJSON for Flex Components and Containers #115 (acoshift)
• Replace format strings of tests from %q to %v #114 (sugyan)
• Retire reviewdog and update travis.yml #113 (sugyan)
• Add ImagemapVideo #112 (galigalikun)
• ci: stop support of Go 1.7 and 1.8 #111 (suzuki-shunsuke)
• Fix not to ignore Hash.write error #110 (shuheiktgw)
• Update CONTRIBUTING.md #107 (syleeeee)
• Lint #105 (suzuki-shunsuke)
• Update supported versions #104 (sugyan)