Revert "Upgrade to TestNG 6.11"
Revert "Upgrade to TestNG 6.11"

TestNG only works with Java 8 or higher while we want to retain
compatibility with Java 7 for now.

Refs cbeust/testng#1377

This reverts commit 7d35b4b4e90650eb7f0c412aa3267558e9a4fbc6.

Graylog2/gelfclient