gemspec: allow fluentd version < 2.0.0

Allow to use a fluentd version < 2.0.0

fluent-plugin-wendelin.gemspec

@@ -11,7 +11,7 @@ Gem::Specification.new do |gem|
     gem.files           = `git ls-files -z`.split("\x0")
     gem.require_paths   = ["lib"]
 
-    gem.add_runtime_dependency "fluentd", "~> 0"
+    gem.add_runtime_dependency "fluentd", "< 2"
     # ? net/http
     # ? openssl
 end