Drop ReferenceSetId from SearchReads

This can be inferred from the context of the search, e.g. I search a given set of ReadGroups which are aligned to ReferenceSets, I shouldn't need to specify the ReferenceSet twice.
Merge branch 'master' of https://github.com/ga4gh/schemas into patch-4

Conflicts:
	src/main/resources/avro/readmethods.avdl
Merge pull request #117 from calbach/patch-4

Drop ReferenceSetId from SearchReads