Move dependencies to the gemspec and lock down versions.

Development dependencies should go in the gemspec when using a Gemfile. Versions
are locked down to prevent failures creeping in as dependencies are updated (so
long as they play nice with semantic versioning). An example of this is the
minitest gem; see issue 164 (rubyzip#164).

Gemfile

@@ -1,7 +1,3 @@
 source 'https://rubygems.org'
 
 gemspec
-gem 'rake'
-gem 'coveralls', :require => false
-gem 'pry'
-gem 'minitest'

rubyzip.gemspec

@@ -16,4 +16,8 @@ spec = Gem::Specification.new do |s|
   s.require_paths         = ['lib']
   s.license               = 'BSD 2-Clause'
   s.required_ruby_version = '>= 1.9.2'
+  s.add_development_dependency 'rake', '~> 10.3'
+  s.add_development_dependency 'pry', '~> 0.10'
+  s.add_development_dependency 'minitest', '~> 5.2.0'
+  s.add_development_dependency 'coveralls', '~> 0.7'
 end