* Loosen pry, pry-byebug depencency
```
Resolving dependencies...
byebug-9.1.0 requires ruby version >= 2.2.0, which is
incompatible with the current version, ruby 2.1.10p492
```
* Adjust nokogiri version constraint for CI
Update appveyor Ruby to 2.3 to work around:
```
Gem::InstallError: nokogiri requires Ruby version < 2.5, >= 2.2.
An error occurred while installing nokogiri (1.8.0), and Bundler cannot continue.
Make sure that `gem install nokogiri -v '1.8.0'` succeeds before bundling.
```
and not 2.4 since:
https://ci.appveyor.com/project/bf4/active-model-serializers/build/1.0.1052-fix_ci/job/0q3itabsnvnxr83u
```
nokogiri-1.6.8.1-x86-mingw32 requires ruby version < 2.4, which is incompatible with the current version, ruby 2.4.1p111
```
* Include rails gem in Gemfile
(For Rails5)
In Rails5, checking for Rails::Railtie is better
* Rails5 test env requires Rails.application.class.name
rails-42d09f6b49da/railties/lib/rails/application.rb
```ruby
def secret_key_base
if Rails.env.test? || Rails.env.development?
Digest::MD5.hexdigest self.class.name
```
* Reformat exclude matrix to be easier to read
* Simplify jruby-travis config per rails/rails
* Organize .travis.yml per rails/rails
* Allow JRuby failure on Rails 5+; try rails-5 db adapter branch
https://github.com/jruby/activerecord-jdbc-adapter/issues/708
```
uninitialized constant ActiveRecord::ConnectionAdapters::Column::Format
```
see https://travis-ci.org/rails-api/active_model_serializers/jobs/277112008
The rubocop only runs in the CI this way a contributor probably will see a
rubocop offense only in the CI.
Running the rubocop in the default rake task we have more chance that a offense
be get in the local machine.
