removes travis-ci.org references since it's out of service since 2021-06-15 (#238)

.travis.yml

@@ -1,23 +0,0 @@
-language: ruby
-os: linux
-rvm:
-  - 2.6
-  - 2.7
-  - 3.0
-  - ruby-head
-arch:
-  - amd64
-  - arm64
-jobs:
-  include:
-    - rvm: 2.6
-      os: osx
-      osx_image: xcode11.3
-    - rvm: 2.6
-      os: osx
-      osx_image: xcode12.2
-    - rvm: 2.7
-      os: osx
-      osx_image: xcode12.2
-dist: xenial
-cache: bundler

README.md

@@ -1,6 +1,6 @@
 # MiniRacer
 
-[![Build Status](https://travis-ci.org/rubyjs/mini_racer.svg?branch=master)](https://travis-ci.org/rubyjs/mini_racer)
+[![Test](https://github.com/rubyjs/mini_racer/actions/workflows/ci.yml/badge.svg)](https://github.com/rubyjs/mini_racer/actions/workflows/ci.yml)
 
 Minimal, modern embedded V8 for Ruby.
 
@@ -434,20 +434,6 @@ If you have a problem installing mini_racer, please consider the following steps
 * make sure to recompile/reinstall `mini_racer` and `libv8-node` after system upgrades (for example via `gem uninstall --all mini_racer libv8-node`)
 * make sure you are on the latest patch/teeny version of a supported Ruby branch
 
-
-## Travis-ci
-
-To install `mini-racer` you will need a version of GCC that supports C++11 (GCC 6.3) this is included by default in ubuntu trusty based images.
-
-Travis today ships by default with a precise based image. Precise Pangolin (12.04 LTS) was first released in August 2012. Even though you can install GCC 6.3 on precise the simpler approach is to opt for the trusty based image.
-
-Add this to your .travis.yml file:
-
-```
-- sudo: required
-- dist: trusty
-```
-
 ## Similar Projects
 
 ### therubyracer