Fix rake version incompatibility by pinning to rake 10.5 on ruby < 1.9.3

See: https://github.com/travis-ci/travis.rb/issues/380

docile.gemspec

@@ -22,7 +22,8 @@ Gem::Specification.new do |s|
   s.required_ruby_version = '>= 1.8.7'
 
   # Run rspec tests from rake
-  s.add_development_dependency 'rake'
+  s.add_development_dependency 'rake', '~> 10.5.0' if on_less_than_1_9_3? # Pin compatible rake on old rubies, see: https://github.com/travis-ci/travis.rb/issues/380
+  s.add_development_dependency 'rake'              unless on_less_than_1_9_3?
   s.add_development_dependency 'rspec', '~> 3.0.0'
 
   # NOTE: needed for Travis builds on 1.8, but can't yet reproduce failure locally

on_what.rb

@@ -12,3 +12,7 @@ end
 def on_1_8?
   RUBY_VERSION.start_with? '1.8'
 end
+
+def on_less_than_1_9_3?
+  RUBY_VERSION < '1.9.3'
+end