Update to Swift 0.15.6 to fix non-system classloaders
<dep.testng.version>6.10</dep.testng.version>
<dep.assertj-core.version>3.8.0</dep.assertj-core.version>
<dep.nifty.version>0.15.1</dep.nifty.version>
- <dep.swift.version>0.15.5</dep.swift.version>
+ <dep.swift.version>0.15.6</dep.swift.version>
<!-- use a fractional hour timezone offset for tests -->
<air.test.timezone>Asia/Katmandu</air.test.timezone>
