Remove warning 'WARNING: open-ended dependency'

Author: Lucca Kaiser

mysql_blob_streaming.gemspec

@@ -13,7 +13,7 @@ Gem::Specification.new do |spec|
   EOF
   spec.license = 'MIT'
 
-  spec.add_dependency "mysql2", ">= 0.5.5"
+  spec.add_dependency "mysql2", "~> 0.5", ">= 0.5.5"
   spec.required_ruby_version = ">= 2.0.0"
 
   spec.files = Dir["lib/**/*.rb", "ext/**/*.{c,h,rb}", "README.markdown"]