Avoid simplecov and truffleruby issue

Author: pitbulk

ruby-saml.gemspec

@@ -51,7 +51,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency('mocha',    '~> 0.14')
   s.add_development_dependency('rake',     '~> 10')
   s.add_development_dependency('shoulda',  '~> 2.11')
-  s.add_development_dependency('simplecov')
+  s.add_development_dependency('simplecov', '<0.22.0')
   s.add_development_dependency('systemu',  '~> 2')
 
   if RUBY_VERSION < '2.1'