Locked rspec-rails to 3.x in the .gemspec

(Since Gemfile.lock isn't currently under version control, and probably can't be since RAILS_VERSION
is dynamic.)

Without this, it will install the latest version, 4.0, which uses &. not available in older Rubies, causing errors like this:

   ruby/2.1.0/gems/rspec-rails-4.0.0/lib/rspec/rails/example/mailer_example_group.rb:25: syntax error, unexpected '.'
               options&.each { |key, value| default_u...

xray-rails.gemspec

@@ -16,7 +16,7 @@ Gem::Specification.new do |gem|
 
   gem.add_dependency 'rails', '>= 3.1.0'
 
-  gem.add_development_dependency 'rspec-rails'
+  gem.add_development_dependency 'rspec-rails', '~> 3.8'
 
   # Required for the dummy Rails app in spec/dummy
   gem.add_development_dependency 'sqlite3'