Don't define VERSION and LEVEL_MAP if already defined

This relies on #7 being merged.

When loading in Rails environment, I've been seeing warnings like:

  /usr/lib/ruby/gems/1.8/gems/SyslogLogger-2.0/lib/syslog/logger.rb:61: warning: already initialized constant VERSION
  /usr/lib/ruby/gems/1.8/gems/SyslogLogger-2.0/lib/syslog/logger.rb:77: warning: already initialized constant LEVEL_MAP

I could imagine that happening in development with reloading, but this
was coming up in staging/production, so I'm not sure how this ends up
getting required and loaded multiple times, but this patch does prevent
redefining.