Update gemspec file to latest style standards (#21)

- Remove test_files attribute since it's deprecated and no longer used.
rubygems/guides#90 (comment)
- Use Dir[] instead of git ls-files to specify gem files, in order to improve readability and exclude unnecessary files.
- Don't include development files and RSpec tests in the built gem.

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/).
 
 #### Changed
 - Rename attr_reader `conn` to `connection` ([#18](https://github.com/epistrephein/rarbg/pull/18)).
+- Remove unnecessary files and attributes from gemspec ([#21](https://github.com/epistrephein/rarbg/pull/21)).
 - Update version constraints for Rake, following v13.0 release ([#15](https://github.com/epistrephein/rarbg/pull/15)).
 - Update version constraints for RuboCop in order to avoid breaking changes ([#16](https://github.com/epistrephein/rarbg/pull/16)).
 - Tweak and reorder documentation ([#18](https://github.com/epistrephein/rarbg/pull/18)).

rarbg.gemspec

@@ -15,12 +15,6 @@ Gem::Specification.new do |spec|
   spec.homepage      = 'https://github.com/epistrephein/rarbg'
   spec.license       = 'MIT'
 
-  spec.files         = `git ls-files -z`.split("\x0").reject do |f|
-    f.match(%r{^(spec|docs)/})
-  end
-  spec.test_files    = `git ls-files -- {test,spec,features}/*`.split("\n")
-  spec.require_path  = 'lib'
-
   spec.metadata = {
     'bug_tracker_uri'   => 'https://github.com/epistrephein/rarbg/issues',
     'changelog_uri'     => 'https://github.com/epistrephein/rarbg/blob/master/CHANGELOG.md',
@@ -29,6 +23,10 @@ Gem::Specification.new do |spec|
     'source_code_uri'   => 'https://github.com/epistrephein/rarbg'
   }
 
+  spec.files         = Dir['lib/**/*.rb', 'rarbg.gemspec']
+  spec.files        += Dir['README.md', 'CHANGELOG.md', 'LICENSE']
+  spec.require_path  = 'lib'
+
   spec.required_ruby_version = '>= 2.0'
 
   spec.add_runtime_dependency 'faraday', '~> 0.12'