Support for rails 6

[pnghai]

fix Rubyzip conflict with webdriver and selenium driver (rubyzip >=1.2.2) on Rails 6

 In Gemfile: active_admin_import was resolved to 4.2.0, which depends on rubyzip (~> 1.2) selenium-webdriver was resolved to 3.142.7, which depends on rubyzip (>= 1.2.2) webdrivers was resolved to 4.2.0, which depends on rubyzip (>= 1.3.0)

[coveralls]

Coverage increased (+0.003%) to 97.959% when pulling 3b634f4 on pnghai:master into 07cee9c on activeadmin-plugins:master.

[pnghai]

@Fivell Please consider this when you have time

[Fivell]

@pnghai thanks