Select first available Rack handler given an `Array` of server names. Raises `LoadError` if no handler was found.
> pick ['thin', 'webrick'] => Rack::Handler::WEBrick
Transforms server-name constants to their canonical form as filenames, then tries to require them but silences the LoadError if not found
Naming convention:
Foo # => 'foo' FooBar # => 'foo_bar.rb' FooBAR # => 'foobar.rb' FOObar # => 'foobar.rb' FOOBAR # => 'foobar.rb' FooBarBaz # => 'foo_bar_baz.rb'