drop support for ruby 2.6

fix ci badge

Author: seuros

Diff for: .github/workflows/ci.yml

@@ -2,6 +2,9 @@
 name: CI
 
 on:
+  push:
+    branches:
+      - master
   pull_request:
     branches:
       - master

Diff for: README.md

@@ -5,8 +5,7 @@
 Common applications include modeling hierarchical data, like tags, threaded comments, page graphs in CMSes,
 and tracking user referrals.
 
-[![Join the chat at https://gitter.im/closure_tree/Lobby](https://badges.gitter.im/closure_tree/Lobby.svg)](https://gitter.im/closure_tree/Lobby?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
-[![Build Status](https://api.travis-ci.org/ClosureTree/closure_tree.svg?branch=master)](http://travis-ci.org/ClosureTree/closure_tree)
+[![CI](https://github.com/ClosureTree/closure_tree/actions/workflows/ci.yml/badge.svg?branch=master)](https://github.com/ClosureTree/closure_tree/actions/workflows/ci.yml)
 [![Gem Version](https://badge.fury.io/rb/closure_tree.svg)](https://badge.fury.io/rb/closure_tree)
 
 Dramatically more performant than

Diff for: closure_tree.gemspec

@@ -18,7 +18,7 @@ Gem::Specification.new do |gem|
   end
 
   gem.test_files  = gem.files.grep(%r{^spec/})
-  gem.required_ruby_version = '>= 2.6.8'
+  gem.required_ruby_version = '>= 2.7.7'
 
   gem.add_runtime_dependency 'activerecord', '>= 6.0.0'
   gem.add_runtime_dependency 'with_advisory_lock', '>= 4.0.0'