[CI] Pinned Rake to version 10.x to prevent clashes with the Yard gem

The builds have been failing on Travis with:

      cd /home/travis/build/elastic/elasticsearch-rails/elasticsearch-model && unset BUNDLE_GEMFILE && bundle exec rake test:unit
      rm -rf tmp/reports
      rake aborted!
      NoMethodError: undefined method `last_comment' for #<Rake::Application:0x00000000a9c478>
      /home/travis/.rvm/gems/ruby-1.9.3-p551/gems/yard-0.8.7.6/lib/yard/rake/yardoc_task.rb:69:in `define'

Related: elastic#538

Author: karmi

Gemfile

@@ -1,7 +1,7 @@
 source 'https://rubygems.org'
 
 gem "bundler", "~> 1"
-gem "rake"
+gem "rake", "< 11.0"
 
 gem 'elasticsearch'
 gem 'elasticsearch-extensions'

elasticsearch-model/elasticsearch-model.gemspec

@@ -28,7 +28,7 @@ Gem::Specification.new do |s|
   s.add_dependency "hashie"
 
   s.add_development_dependency "bundler", "~> 1.3"
-  s.add_development_dependency "rake"
+  s.add_development_dependency "rake", "< 11.0"
 
   s.add_development_dependency "elasticsearch-extensions"
 

elasticsearch-persistence/elasticsearch-persistence.gemspec

@@ -31,7 +31,7 @@ Gem::Specification.new do |s|
   s.add_dependency "virtus"
 
   s.add_development_dependency "bundler", "~> 1.5"
-  s.add_development_dependency "rake"
+  s.add_development_dependency "rake", "< 11.0"
 
   s.add_development_dependency "oj"
 

elasticsearch-rails/elasticsearch-rails.gemspec

@@ -24,7 +24,7 @@ Gem::Specification.new do |s|
   s.required_ruby_version = ">= 1.9.3"
 
   s.add_development_dependency "bundler", "~> 1.3"
-  s.add_development_dependency "rake"
+  s.add_development_dependency "rake", "< 11.0"
 
   s.add_development_dependency "elasticsearch-extensions"
   s.add_development_dependency "elasticsearch-model"