Fix withAllowedTimestampSkew deprecation warning

[nevillelyh]

Need to look at the implications.

def timestampBy(f: T => Instant, allowedTimestampSkew: Duration = Duration.ZERO): SCollection[T] =
    this.applyTransform(WithTimestamps.of(Functions.serializableFn(f))
      .withAllowedTimestampSkew(allowedTimestampSkew))

https://issues.apache.org/jira/browse/BEAM-644