Sets the default instance count for mysql broker to 2.

[finishes #60425858]
Bump src/cf-mysql-broker

Bump cloudfoundry/cf-mysql-broker:
  David Sabeti and Ruthie Byers:
     Use up/down syntax for migrations so we can run ActiveRecord code in migration ( Travis Success: https://travis-ci.org/cloudfoundry/cf-mysql-broker/builds/34768368 )
Adds documentation for how to set up round robin dns

[#78300870]