Update upstream source from tag 'upstream/2.6.1'

Update to upstream version '2.6.1'
with Debian dir 4371f56

Author: sorah

.travis.yml

@@ -378,6 +378,7 @@ branches:
     - ruby_2_3
     - ruby_2_4
     - ruby_2_5
+    - ruby_2_6
     - /^feature\//
     - /^bug\//
     - /^travis-/