Fix missing CSS-styling in production mode

[workgena]

This PR is intended to fix styling error:

Before	After

• Add Rails 5,2 and ActiveAdmin 1.3 to TravisCI
• Fix spec with new CSS injection
• Update README

[workgena]

@Fivell can you please check out why Travis CI does not run?

Expected — Waiting for status to be reported

[Fivell]

@workgena this is github issue , they have some problems now https://twitter.com/githubstatus

[workgena]

Looks like we have deprecated(not supported) configuration of Travis:

https://docs.travis-ci.com/user/legacy-services-to-github-apps-migration-guide/

As of May 2, 2018 we are moving toward having repositories integrated via a GitHub App instead of GitHub Services which will no longer be supported as of October 1, 2018.

[Fivell]

@workgena are u able to migrate ?

[workgena]

@Fivell not, I can't login to Travis "activeadmin-plugins" namespace

[coveralls]

Coverage decreased (-0.5%) to 94.774% when pulling a7731bf on workgena:compatibilty_with_activeadmin_1.3 into 081593e on activeadmin-plugins:master.

[workgena]

The problem was with bin/rails assets:precompile. It loses the content of "jquery.xdan.datetimepicker.css". The only working solution, I managed to find, is to use ActiveAdmin register_stylesheet method. It is DEPRECATED and will be removed in 2.0 version. But, for now, it works correctly. So I propose to use it.

# config/initializers/active_admin_datetimepicker.rb
ActiveAdmin.setup do |config|
  config.register_stylesheet 'jquery.xdan.datetimepicker.css'
end

[Fivell]

@workgena 👍 feel free to bump new version

[benjaminwood]

FYI, it's not released yet but active admin has undeprecated register_stylesheet

activeadmin/activeadmin#5662

Guess it paid off to wait until active_admin provided a 'solution' 😄