Update gemspec dependencies, particularly for mysql2
Update gemspec dependencies, particularly for mysql2

to fix travis-ci not passing with Rails 5.0.0/master.

guilleva/ransack,avit/ransack,Eric-Guo/ransack,reinteractive/ransack,mariorcardoso/ransack,outstand/ransack,activerecord-hackery/ransack,johan--/ransack,roman-franko/ransack,AndrewSwerlick/ransack,Mario245/ransack,martndemus/ransack,laserlemon/ransack