Update marcel.gemspec

* Suppress warnings when running "gem build marcel.gemspec".
* Typo filenames => file names.

Author: junaruga

marcel.gemspec

@@ -8,7 +8,7 @@ Gem::Specification.new do |spec|
   spec.version       = Marcel::VERSION
   spec.authors       = ['Tom Ward']
   spec.email         = ['[email protected]']
-  spec.summary       = %q{Simple mime type detection using magic numbers, filenames, and extensions}
+  spec.summary       = %q{Simple mime type detection using magic numbers, file names, and extensions}
   spec.homepage      = 'https://github.com/basecamp/marcel'
   spec.license       = 'MIT'
 
@@ -21,8 +21,8 @@ Gem::Specification.new do |spec|
 
   spec.add_dependency 'mimemagic', '~> 0.3.2'
 
-  spec.add_development_dependency 'minitest'
+  spec.add_development_dependency 'minitest', '~> 5.11'
   spec.add_development_dependency 'bundler', '~> 1.7'
   spec.add_development_dependency 'rake', '~> 10.0'
-  spec.add_development_dependency 'rack', '~> 2.0.1'
+  spec.add_development_dependency 'rack', '~> 2.0'
 end