Bumps SwiftyJSON dependency version to match the version Kitura uses

[nesty]

Fixes swift build infinity loop when building projects with Kitura (requires version 17) and GRMustache.swift (required version 16.0).

[helenmasters]

Hi @nesty - I've been looking at templating and noticed that this PR has been outstanding
for a long time, apologies :-(

Given that you would need to have signed the CLA (Contributor License Agreement) before we would have been able to merge your request I took the quicker route of creating a new PR #12 and merging that instead. Please do contribute to our other IBM-Swift repositories (which are not at IBM-Swift-Sunset) where the CLA signing is now an automated process. Once again my apologies that this issue has been on the queue for so long.

Hopefully this issue can now be closed as Kitura no longer depends on SwiftyJSON, but given that SwiftMetrics still does you would have had the same problem there so I have tested the fix and you can now use SwiftMetrics successfully alongside GRMustache.

Regards Helen