Update README.md

Bats feils have .bats as extension. reading on bats git page (https://github.com/sstephenson/bats) I saw all test files are .bats. I renamed the tomcat.rb file to bats (it worked "2 tests, 0 failures")  and back to rb (it showed "0 tests, 0 failures") - even though the file was uploaded in both cases.