* lib/prime.rb (Prime#prime?): negative numbers can't be primes
  by definition. reported by Ivan Kataitsev. [Bug #7395]
* test/test_prime.rb: add test.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45878 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-05-09

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45879 b2dd03c8-39d4-4d8f-98ff-823fe69b080e