Reduce gem size by excluding test files (#397)

Only ship lib/ and essential files (CHANGELOG, LICENSE, README) instead of everything tracked by git.

Author: yuri-zubov

CHANGELOG.md

@@ -3,6 +3,7 @@
 #### Features
 
 * [#400](https://github.com/ruby-grape/grape-entity/pull/400): Switch to danger-pr-comment for PR checks - [@numbata](https://github.com/numbata).
+* [#397](https://github.com/ruby-grape/grape-entity/pull/397): Reduce gem size by excluding test files - [@yuri-zubov](https://github.com/yuri-zubov).
 * Your contribution here.
 
 #### Fixes

grape-entity.gemspec

@@ -18,7 +18,6 @@ Gem::Specification.new do |s|
 
   s.add_dependency 'activesupport', '>= 3.0.0'
 
-  s.files         = `git ls-files`.split("\n")
-  s.test_files    = `git ls-files -- {test,spec}/*`.split("\n")
+  s.files         = `git ls-files lib`.split("\n") + ['CHANGELOG.md', 'LICENSE', 'README.md']
   s.require_paths = ['lib']
 end