Bump Syslog::Logger::VERSION to 2.1.0.

* ext/syslog/lib/syslog/logger.rb (Syslog::Logger::VERSION): Bump
  the VERSION to 2.1.0. [ruby-core:64483] [Bug #10159]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48208 b2dd03c8-39d4-4d8f-98ff-823fe69b080e