Lock rake to 10.1 for Ruby 1.8.7

Rake has gone 1.9.3+ from 10.2 onwards so the Gemfile for 1.8.7
specifies this new constraint.

Gemfile.1.8.7

@@ -6,6 +6,7 @@ gem 'mime-types', '~>1.16'
 group :development, :test do
   # This is here because gemspec doesn't support require: false
   gem 'coveralls', :require => false
+  gem 'rake', '~> 10.1.0'
 end
 
 gemspec