Unbreak Travis for Rails 5.x

.travis.yml

@@ -4,7 +4,6 @@ addons:
 rvm:
   - 2.3.6
 gemfile:
-  - rails_4_2.gemfile
   - rails_5_0.gemfile
   - rails_5_1.gemfile
   - rails_5_2.gemfile

README.md

@@ -17,7 +17,7 @@ user_1.posts.union(user_2.posts).union(Post.published)
 user_1.posts.union_all(user_2.posts)
 ```
 
-ActiveRecordUnion is tested against Rails 4.2, 5.0, 5.1, and 5.2. It may or may not work on Rails 4.0/4.1.
+ActiveRecordUnion is tested against Rails 5.0, 5.1, and 5.2. It should also work on Rails 4.2. It may or may not work on Rails 4.0/4.1.
 
 If you are using Postgres, you might alternatively check out [ActiveRecordExtended](https://github.com/georgekaraszi/ActiveRecordExtended) which includes support for unions as well as other goodies.
 

rails_4_2.gemfile

@@ -1,11 +0,0 @@
-source 'https://rubygems.org'
-
-# Specify your gem's dependencies in active_record_union.gemspec
-gemspec
-
-gem 'rails', '~> 4.2.7'
-
-# On Rails < 5.2, only pg < v1 is supported. See:
-# https://github.com/rails/rails/pull/31671
-# https://bitbucket.org/ged/ruby-pg/issues/270/pg-100-x64-mingw32-rails-server-not-start
-gem 'pg', '~> 0.21'

rails_5_0.gemfile

@@ -9,3 +9,5 @@ gem 'rails', '~> 5.0.0'
 # https://github.com/rails/rails/pull/31671
 # https://bitbucket.org/ged/ruby-pg/issues/270/pg-100-x64-mingw32-rails-server-not-start
 gem 'pg', '~> 0.21'
+
+gem 'sqlite3'

rails_5_1.gemfile

@@ -9,3 +9,5 @@ gem 'rails', '~> 5.1.0'
 # https://github.com/rails/rails/pull/31671
 # https://bitbucket.org/ged/ruby-pg/issues/270/pg-100-x64-mingw32-rails-server-not-start
 gem 'pg', '~> 0.21'
+
+gem 'sqlite3'

rails_5_2.gemfile

@@ -4,4 +4,8 @@ source 'https://rubygems.org'
 gemspec
 
 # pg v1.0+ compatibility, https://github.com/rails/rails/pull/31671:
-gem 'rails', '~> 5.2.0.beta2', git: 'https://github.com/rails/rails', ref: 'f1af27fd9d9101684b26d0dcf2028859d67bec1f'
+gem 'rails', '~> 5.2.0'
+
+gem 'pg'
+
+gem 'sqlite3'