Minimize artifacts deployed to maven central

Move shaded-slf4j-impl into this repo so deploying it to maven central is
no longer needed.

Don't deploy glowroot-package to maven central.

Don't shade glowroot-core when deploying it to maven central.
Fix maven build warning
Fix test failure on OpenJDK 6
Fix sporadic test failure
Add checker and sonar builds to travis-ci
Fix sporadic travis-ci build failure
Fix sporadic test failure