Railties CI against Ruby 3.0.0-dev has been failing
since https://buildkite.com/rails/rails/builds/73338#51165eef-b631-4f9c-8e98-fc210e71e6d1

This pull request addresses the following error.

```ruby
$ ruby -v
ruby 3.0.0dev (2020-12-13T04:39:57Z master c7530f0d56) [x86_64-linux]
$ ruby -Itest -Ilib -I../activesupport/lib -I../actionpack/lib -I../actionview/lib -I../activemodel/lib test/commands/server_test.rb
... snip ...
Run options: --seed 54404

.E

Error:
Rails::Command::ServerCommandTest#test_served_url:
LoadError: cannot load such file -- webrick
    test/commands/server_test.rb:292:in `test_served_url'

rails test test/commands/server_test.rb:289

........................

Finished in 0.303746s, 85.5977 runs/s, 177.7799 assertions/s.
26 runs, 54 assertions, 0 failures, 1 errors, 0 skips
$
```

Related to
ruby/ruby#3729
https://bugs.ruby-lang.org/issues/17303

webrick has been removed from stdlib in Ruby 3.0. This change allows
specs to start with a pristine 3.0 gemset

Ref:
- https://bugs.ruby-lang.org/issues/17303
- ruby/ruby#3729

webrick has been removed from stdlib in Ruby 3.0. This change allows
specs to start with a pristine 3.0 gemset

Ref:
- https://bugs.ruby-lang.org/issues/17303
- ruby/ruby#3729

webrick has been removed from stdlib in Ruby 3.0. This change allows
specs to start with a pristine 3.0 gemset

Ref:
- https://bugs.ruby-lang.org/issues/17303
- ruby/ruby#3729

Webrick was removed from the Ruby stdlib in late 2020 (see
https://bugs.ruby-lang.org/issues/17303 &
ruby/ruby#3729). This adds it as a development
dependency for starting up the simple web server for the sample project.

Webrick seems like a fine fit for a simple server like this.