Fix test suite.

This MR fixes a few issues with PrometheusSink's test suite.

Changelog:

• Fixed a syntax error in src/test/scala/com/banzaicloud/spark/metrics/sink/PrometheusSinkSuite.scala.
• Fixed missing properties in src/test/scala/org/apache/spark/banzaicloud/metrics/sink/PrometheusSinkSuite.scala. spark.app.name and spark.app.id must be present in other to initialise a PrometheusSink object
• build.sbt add a dependency on org.eclipse.jetty, that was not previously resolved.