Upgrade to sbt 0.13.7.

Macro paradise is kept at 0.13.5 because otherwise it blows up with

[paradise-210] java.lang.NullPointerException
[paradise-210] 	at xsbt.Dependency$ExtractDependenciesByMemberRefTraverser$$anonfun$1.isDefinedAt(Dependency.scala:142)
[paradise-210] 	at xsbt.Dependency$ExtractDependenciesByMemberRefTraverser$$anonfun$1.isDefinedAt(Dependency.scala:141)
[paradise-210] 	at xsbt.Dependency$CollectTypeTraverser.traverse(Dependency.scala:89)

Fixes #80.
Revert "Revert akka-libs-210 to 49d90828517bd03dbb52e878430c4af47df33992"

This reverts commit c8e7d1641fb39a778d1beeb5e66b7500e542f190. We upgraded
to sbt 0.13.7 in previous commit so the workaround from
c8e7d1641fb39a778d1beeb5e66b7500e542f190 is not needed anymore.
Merge pull request #81 from gkossakowski/sbt-upgrade-to-0.13.7

Upgrade sbt to 0.13.7