Drop support for Ruby 2.2

Ruby 2.2 has been EOL since June 2018. Supporting it requires additional
effort in our test environment, so drop support.

Author: sds

.rubocop.yml

@@ -1,5 +1,5 @@
 AllCops:
-  TargetRubyVersion: 2.2
+  TargetRubyVersion: 2.3
 
 Layout/ClosingParenthesisIndentation:
   Enabled: false

.travis.yml

@@ -3,25 +3,18 @@ language: ruby
 cache: bundler
 
 rvm:
-  - 2.2
-  - 2.3.7
-  - 2.4.4
-  - 2.5.1
-  - 2.6
+  - 2.3.8
+  - 2.4.5
+  - 2.5.5
+  - 2.6.2
 
 before_script:
   - git config --global user.email "travis@travis.ci"
   - git config --global user.name "Travis CI"
 
 before_install:
-  - |
-    export RVM_CURRENT=`rvm current|cut -c6-8`
-    if [ "${RVM_CURRENT}" == "2.2" ]; then
-      gem --version
-    else
-      gem update --system
-    fi
-  - gem install bundler:'< 2'
+  - gem update --system
+  - gem install bundler
 
 script:
   - bundle exec rspec

README.md

@@ -54,7 +54,7 @@ any Ruby code.
 
 This project aims to support the following Ruby runtimes on both \*nix and Windows:
 
-* MRI 2.2+
+* MRI 2.3+
 
 ### Dependencies
 

overcommit.gemspec

@@ -25,7 +25,7 @@ Gem::Specification.new do |s|
                             Dir['libexec/**/*'] +
                             Dir['template-dir/**/*']
 
-  s.required_ruby_version = '>= 2.2'
+  s.required_ruby_version = '>= 2.3'
 
   s.add_dependency          'childprocess', '~> 0.6', '>= 0.6.3'
   s.add_dependency          'iniparse', '~> 1.4'