Add support for platform "amazon" (tested)

in accordance with current maintainer, repeatedly@gmail.com.  
As per issue, Unnecessarily strict platform check #18,
  "We don't have a test environment for Amazon Linux and we don't guarantee td-agent works on Amazon Linux.
  If it is OK, please send a PR :)"
Done.   I have been using and testing the mod in question on amazon, centos, and ubuntu over the past week.

thanks!