Reduce Compiler Warnings, Fix Promise Bug, Modernize Scala & SBT

This pair of commits cleans up warnings, fixes an exception that was thrown because of failing an already completed promise, and upgrades to SBT 0.13.7 and scala 2.11.4

Log messages:
Reduce Compiler Warnings & Fix A Bug …
* Turn -feature on in plugins.sbt so feature warnings are printed by sbt
* Fix the deprecation of Default.defaultSettings in build
* Introduce scala language options as needed to reduce warnings
* Fix many deprecated scala macro changes
* Fix a bug in MongoDBSystem.postStop to avoid calling failure on an already completed promise which previously threw IllegalStateException("Promise already completed.")

Modernize Scala & SBT Version …
* Use SBT 0.13.7 - faster dependency resolution, especially for locally published artifacts
* Use Scala 2.11.4