The native websocket_mask extension should be an optional dependency

[SpComb]

It should be possible to depend on and use the kontena-websocket-client gem without requiring a compiler for native ruby extensions when installing the gem:

~ $ gem install kontena-websocket-client
Fetching: websocket-extensions-0.1.2.gem (100%)
Successfully installed websocket-extensions-0.1.2
Fetching: websocket-driver-0.6.5.gem (100%)
Building native extensions.  This could take a while...
ERROR:  Error installing kontena-websocket-client:
	ERROR: Failed to build gem native extension.

    current directory: /home/kontena/.gem/ruby/2.3.0/gems/websocket-driver-0.6.5/ext/websocket-driver
/usr/bin/ruby -r ./siteconf20170725-9-4pljei.rb extconf.rb
mkmf.rb can't find header files for ruby at /usr/lib/ruby/include/ruby.h

extconf failed, exit code 1

Gem files will remain installed in /home/kontena/.gem/ruby/2.3.0/gems/websocket-driver-0.6.5 for inspection.
Results logged to /home/kontena/.gem/ruby/2.3.0/extensions/x86_64-linux/2.3.0/websocket-driver-0.6.5/gem_make.out

This currently fails on the websocket-driver gem'swebsocket_mask extension. The native websocket_mask extension is optional: WebSocket::Driver will already fall back to the pure-ruby websocket/mask implementation (faye/websocket-driver-ruby#48).