Upgrade versions on important dependencies.

[lloydmeta]

• Bump Play to 2.4.6 -> https://www.playframework.com/changelog
• Upgrade Apache HttpClient to 4.5.1 -> force 4.3.x behaviour by always revalidating connections for safety
• Upgrade Shade to 1.7.2; latest release -> minor change in client instantiation https://github.com/alexandru/shade/blob/master/release-notes/1.7.md
• Remove separate Spymemcached dependency declaration -> Already brought in via Shade
• Upgrade Hystrix, Hystrix Streaming -> A few fixes and improvements https://github.com/Netflix/Hystrix/blob/master/CHANGELOG.md
• Upgrade RxScala to match Hystrix's RxJava dep -> Many fixes and performance improvements RxScala release notes https://github.com/ReactiveX/RxScala/releases, RxJava release notes https://github.com/ReactiveX/RxJava/releases
• Upgrade DBCP2 -> Bug fixes http://commons.apache.org/proper/commons-dbcp/changes-report.html#a2.1.1
• Upgrade some test dependencies (Scalatest upgrade was needed to allow for Apache HTTPClient's upgrade to v4.5.1)
• Remove duplicate declaration of Typesafe releases repo as a resolver in plugins.sbt

[lloydmeta]

The tests added in #188 and #192 were meant to improve confidence when doing this kind of thing.

[xevix]

A quick smoketest seems good. I ran into #194 in the process of running tests, but it also happens in develop so I think it's separate. Merging, nice work on the update.