Remove development and test files from the gem package (#503)

There are several files in the gem package that aren't useful for
downstream projects. Removing these files reduces the gem package
size from 34K to 24K!

cells.gemspec

@@ -19,8 +19,11 @@ Gem::Specification.new do |spec|
   spec.metadata['source_code_uri'] = spec.homepage
   spec.metadata['wiki_uri'] = "#{spec.homepage}/wiki"
 
-  spec.files         = `git ls-files`.split("\n")
-  spec.test_files    = `git ls-files -- {test}/*`.split("\n")
+  spec.files = Dir.chdir(__dir__) do
+    `git ls-files -z`.split("\x0").reject do |file|
+      file.start_with?(*%w[.git Gemfile Rakefile TODO test])
+    end
+  end
   spec.require_paths = ['lib']
   spec.required_ruby_version = '>= 2.2.10'