Drop Rails 4.2 support

Author: deivid-rodriguez

.travis.yml

@@ -6,7 +6,6 @@ rvm:
   - 2.6.3
   - ruby-head
 env:
-  - RAILS_VERSION="~> 4.2.0"
   - RAILS_VERSION="~> 5.0.0"
   - RAILS_VERSION="~> 5.1.0"
   - RAILS_VERSION="~> 5.2.0"
@@ -15,8 +14,6 @@ matrix:
   exclude:
     - rvm: 2.4.6
       env: RAILS_VERSION="~> 6.0.0.rc1"
-    - rvm: 2.6.3
-      env: RAILS_VERSION="~> 4.2.0"
   allow_failures:
     - rvm: ruby-head
   fast_finish: true

README.md

@@ -17,7 +17,7 @@ boot it every time you run a test, rake task or migration.
 ## Compatibility
 
 * Ruby versions: MRI 2.4, MRI 2.5, MRI 2.6
-* Rails versions: 4.2, 5.0, 5.1, 5.2, 6.0 (Spring is installed by default when you do
+* Rails versions: 5.0, 5.1, 5.2, 6.0 (Spring is installed by default when you do
   `rails new` to generate your application)
 
 Spring makes extensive use of `Process.fork`, so won't be able to

lib/spring/application.rb

@@ -91,8 +91,8 @@ def preload
 
       require Spring.application_root_path.join("config", "application")
 
-      unless Rails.respond_to?(:gem_version) && Rails.gem_version >= Gem::Version.new('4.2.0')
-        raise "Spring only supports Rails >= 4.2.0"
+      unless Rails.respond_to?(:gem_version) && Rails.gem_version >= Gem::Version.new('5.0.0')
+        raise "Spring only supports Rails >= 5.0.0"
       end
 
       # config/environments/test.rb will have config.cache_classes = true. However