1.13

Changed

• increase default NUnit runner version from 2.6.4 to 3.9.0
• built with gradle 4.8.1 (about won't work till we migrate out of plugindev)
• take as test assemblies only those containing "test.dll" or "tests.dll" to not require explicit config for projects like "test-framework.dll"

Fixed

• Upgrade gradle-download-task to 3.4.3 to fix 'Invalid cookie expiry' on download
• Allow download of NUnit 3.9 runner