Fix build error `can't modify frozen String`
Fix build error `can't modify frozen String`

Update gem to the latest version (2.6.10).

https://travis-ci.org/emsk/backlog_kit/jobs/200349504
http://blog.rubygems.org/2017/01/20/2.6.9-released.html

emsk/backlog_kit,emsk/backlog_kit