Use secure protocol when running bundle install

This PR suppresses the following warnings.

```console
% bundle install
The git source `git://github.com/cucumber/cucumber-ruby-core.git` uses
the `git` protocol, which transmits data without encryption. Disable
this warning with `bundle config git.allow_insecure true`, or switch to
the `https` protocol to keep your data secure.
The git source `git://github.com/cucumber/cucumber-ruby-wire.git` uses
the `git` protocol, which transmits data without encryption. Disable
this warning with `bundle config git.allow_insecure true`, or switch to
the `https` protocol to keep your data secure.
Fetching git://github.com/cucumber/cucumber-ruby-core.git
Fetching git://github.com/cucumber/cucumber-ruby-wire.Git
```

Refer: rubygems/bundler#4127

Gemfile

@@ -1,16 +1,19 @@
 source 'https://rubygems.org'
+
+git_source(:github) { |repo| "https://github.com/#{repo}.git" }
+
 gemspec
 
 if ENV['CUCUMBER_RUBY_CORE']
   gem 'cucumber-core', path: ENV['CUCUMBER_RUBY_CORE']
 elsif !ENV['CUCUMBER_USE_RELEASED_GEMS']
-  gem 'cucumber-core', git: 'git://github.com/cucumber/cucumber-ruby-core.git'
+  gem 'cucumber-core', github: 'cucumber/cucumber-ruby-core'
 end
 
 if ENV['CUCUMBER_RUBY_WIRE']
   gem 'cucumber-wire', path: ENV['CUCUMBER_RUBY_WIRE']
 elsif !ENV['CUCUMBER_USE_RELEASED_GEMS']
-  gem 'cucumber-wire', git: 'git://github.com/cucumber/cucumber-ruby-wire.git'
+  gem 'cucumber-wire', github: 'cucumber/cucumber-ruby-wire'
 end
 
 gem 'cucumber-expressions', path: ENV['CUCUMBER_EXPRESSIONS_RUBY'] if ENV['CUCUMBER_EXPRESSIONS_RUBY']