Match Rails Ruby version requirements

.ruby-version

@@ -1 +0,0 @@
-2.5.1

.travis.yml

@@ -2,10 +2,17 @@ language: ruby
 sudo: false
 
 rvm:
-  - 2.2
+  - 2.4.4
+  - 2.5.1
+  - ruby-head
 
 cache:
   bundler: true
 
+matrix:
+  allow_failures:
+    - rvm: ruby-head
+  fast_finish: true
+
 script:
   - bundle exec rake

actiontext.gemspec

@@ -13,7 +13,7 @@
   s.homepage = "https://github.com/rails/actiontext"
   s.license  = "MIT"
 
-  s.required_ruby_version = ">= 2.2.2"
+  s.required_ruby_version = ">= 2.4.1"
 
   s.add_dependency "rails", ">= 5.2.0"
   s.add_dependency "nokogiri"