Update DownstreamContext.groovy

'SUCCESS', 'UNSTABLE' and 'FAILURE' were wrongly mapped to 48, 49 and 50 (ascii values of '0', '1' and '2'). This has the effect, that the generated jobs erroneously showed 'FAILURE' for all thresholds.
In the proposed change the threshold names are mapped to their actual integer value instead of their ascii value.