Create `rails@localhost` user on travis ci

Looks like that latest CI failure is due to missing `rails@localhost`.
Creating the user may solve the issue.

https://travis-ci.org/rails/rails/jobs/254996063#L831

ci/travis.rb

@@ -3,6 +3,10 @@
 include FileUtils
 
 commands = [
+  'mysql -e "create user rails@localhost;"',
+  'mysql -e "grant all privileges on activerecord_unittest.* to rails@localhost;"',
+  'mysql -e "grant all privileges on activerecord_unittest2.* to rails@localhost;"',
+  'mysql -e "grant all privileges on inexistent_activerecord_unittest.* to rails@localhost;"',
   'mysql -e "create database activerecord_unittest;"',
   'mysql -e "create database activerecord_unittest2;"',
   'psql  -c "create database activerecord_unittest;" -U postgres',