Update dependencies, enable pruning for vendor/

So, `dep` got an nice new feature to remove tests and non-go files from
`vendor/`, and this brings the size of the vendor directory from ~300MiB
down to ~20MiB. We don that now.

vendor/golang.org/x/sys/unix/openbsd_pledge.go

@@ -13,7 +13,7 @@ import (
 )
 
 const (
-	SYS_PLEDGE = 108
+	_SYS_PLEDGE = 108
 )
 
 // Pledge implements the pledge syscall. For more information see pledge(2).
@@ -30,7 +30,7 @@ func Pledge(promises string, paths []string) error {
 		}
 		pathsUnsafe = unsafe.Pointer(&pathsPtr[0])
 	}
-	_, _, e := syscall.Syscall(SYS_PLEDGE, uintptr(promisesUnsafe), uintptr(pathsUnsafe), 0)
+	_, _, e := syscall.Syscall(_SYS_PLEDGE, uintptr(promisesUnsafe), uintptr(pathsUnsafe), 0)
 	if e != 0 {
 		return e
 	}