Update all dependencies.

* Switch to using kingpin for CLI interface as upstream Prometheus has moved
  that way too.

vendor/github.com/alecthomas/units/README.md

@@ -0,0 +1,11 @@
+# Units - Helpful unit multipliers and functions for Go
+
+The goal of this package is to have functionality similar to the [time](http://golang.org/pkg/time/) package.
+
+It allows for code like this:
+
+```go
+n, err := ParseBase2Bytes("1KB")
+// n == 1024
+n = units.Mebibyte * 512
+```