Fix compiler warnings after migration to Scala 2.11.4

[vkostyukov]

Most of the warning are in finch-json module:

[warn] /home/travis/build/finagle/finch/finch-json/src/main/scala/io/finch/json/Json.scala:63: match may not be exhaustive.
[warn] It would fail on the following input: Nil
[warn]     def loop(path: List[String], outer: Map[String, Any]): Option[A] = path match {
[warn]                                                                        ^
[warn] /home/travis/build/finagle/finch/finch-json/src/main/scala/io/finch/json/Json.scala:132: match may not be exhaustive.
[warn] It would fail on the following input: Nil
[warn]     def loop(path: List[String], value: Any): Map[String, Any] = path match {
[warn]                                                                  ^
[warn] /home/travis/build/finagle/finch/finch-json/src/main/scala/io/finch/json/Json.scala:161: object JSON in package json is deprecated: This object will be removed.
[warn]     JSON.parseFull(s) match {

[vkostyukov]

Fixed by #117.