Remove use of _id field in push replications using a filter

This is to prepare for not allowing underscores in field names
when using the CDT APIs (see following commit).
Don't accept CDTDocumentBody objects with _fields in
Remove xcpretty from travis test runs
Remove NSLog statements from tests

This makes the test runs more readable by reducing noise.
Merge pull request #40 from cloudant/28237-no-underscores-in-body

Don't allow underscore prefixed fields when using CDT APIs (28237)