Reduce gem size (#233)

* Delete two unused images

They were used in the Readme, but no longer refenced since b133a1b.

* Reduce size of images

Lossless optimization of two png files, reducing file size by about 33%.
I used ImageOptim[^1] for this task.

[^1]: https://imageoptim.com/mac

* Build a smaller gem

Removing unnecessary files from the built gem, reducing its size.

Author: bquorning

images/dragons.png

@@ -23,7 +23,9 @@ Gem::Specification.new do |spec|
     "source_code_uri" => "#{spec.homepage}/tree/v#{spec.version}",
     "wiki_uri" => "#{spec.homepage}/wiki"
   }
-  spec.files         = `git ls-files`.split($INPUT_RECORD_SEPARATOR)
+  spec.files         = `git ls-files`.split($INPUT_RECORD_SEPARATOR).reject do |f|
+    f.start_with?("images/", "spec/", "tasks/", "Rakefile")
+  end
   spec.executables   = spec.files.grep(%r{^bin/}).map { |f| File.basename(f) }
   spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
   spec.require_paths = ["lib"]