DRILL-1409: DbVisualizer fails to connect to Drill using embedded jdbc driver
DRILL-1497: Fix test failure in o.a.drill.jdbc.test.TestMetadataDDL
DRILL-1490: Upgrade to ProGaurd 5.0
DRILL-1493: Upgrade ASM to 5.0
DRILL-1492: Update readme with supported versions of Java

* Keeping the supported Java version to 7 until we get all issues around Java 8 resolved.