Upgrade testNG version

This is the highest version that is still java 8 compatible.
It partially solves some issues with testing, where testNG would crash
when trying to execute `@AfterClass` methods.

Author: Bouncheck

Diff for: pom.xml

@@ -79,7 +79,7 @@
         <!-- more recent versions of pax-exam require JDK7+ -->
         <pax-exam.version>3.6.0</pax-exam.version>
         <url.version>2.4.0</url.version>
-        <testng.version>6.8.8</testng.version>
+        <testng.version>7.5.1</testng.version>
         <assertj.version>1.7.0</assertj.version>
         <mockito.version>1.10.8</mockito.version>
         <wiremock.version>2.25.0</wiremock.version>