[test] keep the rake_test MRI runnable (fix old Ruby compatibility)

Author: kares

test/db/postgresql/rake_test.rb

@@ -140,7 +140,7 @@ def psql(args)
     `#{PSQL_EXE} #{args}`
   end
 
-  if ActiveRecord::VERSION::STRING.start_with?('4.2') && JRUBY_VERSION.start_with?('1.7') rescue nil
+  if ActiveRecord::VERSION::STRING.start_with?('4.2') && defined?(JRUBY_VERSION) && JRUBY_VERSION.start_with?('1.7')
     warn "NOTE: patching structure_dump due on JRuby #{JRUBY_VERSION}"
     # NOTE: a hack around structure_dump failing due system(cmd, *args) on 1.7 (not going to get fixed)
     #   RuntimeError: failed to execute: