Update typescript def to define return type for to
Update typescript def to define return type for to

Using TS 3.0.3 receive this error on build when being strict: 
`error TS7010: 'to', which lacks return-type annotation, implicitly has an 'any' return type.`

This fix ensures that StreamDSL is returned
krystianity/kafka-streams,krystianity/kafka-streams