Add support for Ruby 2.6 to the install script

Author: cm-yoshida-takafumi

bin/install

@@ -86,7 +86,7 @@ EOF
   end
 
   def supported_ruby_versions
-    ['2.5', '2.4', '2.3', '2.2', '2.1', '2.0']
+    ['2.6', '2.5', '2.4', '2.3', '2.2', '2.1', '2.0']
   end
 
   # check ruby version, only version 2.x works