Remove Travis CI

Re: #60

Good bye to Travis CI! You were very appreciated.

But today, supporting only Github Actions for CI will enable
some additional goals, as well as speeding up builds.

Gemfile

@@ -1,6 +1,6 @@
 source "https://rubygems.org"
 
-# Travis-only dependencies go here
+# CI-only dependencies go here
 if ENV["CI"] == "true" && RUBY_ENGINE == "ruby"
   gem "codecov", require: false, group: "test"
 end

README.md

@@ -8,7 +8,6 @@
 [![Docs Coverage](http://inch-ci.org/github/ms-ati/docile.png)](http://inch-ci.org/github/ms-ati/docile)
 
 [![Build Status](https://github.com/ms-ati/docile/actions/workflows/main.yml/badge.svg)](https://github.com/ms-ati/docile/actions/workflows/main.yml)
-[![Build Status](https://img.shields.io/travis/ms-ati/docile/master.svg)](https://travis-ci.org/ms-ati/docile)
 [![Code Coverage](https://img.shields.io/codecov/c/github/ms-ati/docile.svg)](https://codecov.io/github/ms-ati/docile)
 [![Maintainability](https://api.codeclimate.com/v1/badges/79ca631bc123f7b83b34/maintainability)](https://codeclimate.com/github/ms-ati/docile/maintainability)
 
@@ -354,8 +353,7 @@ $ gem install docile
 
 Works on [all currently supported ruby versions](https://github.com/ms-ati/docile/blob/master/.github/workflows/main.yml),
 or so [Github Actions](https://github.com/ms-ati/docile/actions)
-and [Travis CI](https://travis-ci.org/ms-ati/docile)
-tell us.
+tells us.
 
 Used by some pretty cool gems to implement their DSLs, notably including
 [SimpleCov](https://github.com/colszowka/simplecov). Keep an eye out for new

spec/spec_helper.rb

@@ -3,7 +3,7 @@
   require "simplecov"
   SimpleCov.start do
     add_filter "/spec/"      # exclude test code
-    add_filter "/vendor/"    # exclude gems which are vendored on Travis CI
+    add_filter "/vendor/"    # exclude gems which are cached in CI
   end
 
   # On CI we publish coverage to codecov.io, except on JRuby and TruffleRuby