DEV: bump required ruby version to >= 2.6 (#214)

* bump required ruby version to 2.6
* removes Ruby 2.5 from CI

.github/workflows/ci.yml

@@ -29,7 +29,6 @@ jobs:
       fail-fast: false
       matrix:
         ruby:
-          - '2.5'
           - '2.6'
           - '2.7'
           - '3.0'

.travis.yml

@@ -1,7 +1,6 @@
 language: ruby
 os: linux
 rvm:
-  - 2.5
   - 2.6
   - 2.7
   - 3.0
@@ -11,9 +10,6 @@ arch:
   - arm64
 jobs:
   include:
-    - rvm: 2.5
-      os: osx
-      osx_image: xcode9.4
     - rvm: 2.6
       os: osx
       osx_image: xcode11.3

mini_racer.gemspec

@@ -35,5 +35,5 @@ Gem::Specification.new do |spec|
 
   spec.extensions = ["ext/mini_racer_loader/extconf.rb", "ext/mini_racer_extension/extconf.rb"]
 
-  spec.required_ruby_version = '>= 2.5'
+  spec.required_ruby_version = '>= 2.6'
 end