Fixes#8468
If a user wished to bump the `engine_version` of an RDS instance,
Terraform was not sending `allow_major_version_upgrade` to the API
*unless* that value also changed at the same time. This caused the
following error from RDS API:
```
* aws_db_instance.bar: Error modifying DB Instance
* tf-20160825101420910562798obb: InvalidParameterCombination: The
* AllowMajorVersionUpgrade flag must be present when upgrading to a new
* major version.
status code: 400, request id: 20e36364-6ab0-11e6-b794-51f12f4135f1
```
This change will always send the `allow_major_version_upgrade` flag to
the API when the `engine_version` changes.
This still relies on the user setting the correct value i.e. if they are
upgrading from postgres 0.4.7 -> 9.5.2 then the config will need to set
the `allow_major_version_upgrade` flag to be `true`
0adc1fc4b2