Bump up version to v2.6.0

Signed-off-by: Daijiro Fukuda <fukuda@clear-code.com>

Author: daipom

fluent-plugin-concat.gemspec

@@ -4,7 +4,7 @@ $LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
 
 Gem::Specification.new do |spec|
   spec.name          = "fluent-plugin-concat"
-  spec.version       = "2.5.0"
+  spec.version       = "2.6.0"
   spec.authors       = ["Kenji Okimoto"]
   spec.email         = ["okimoto@clear-code.com"]