Bazel: Consume versions directly from lib:versions.bzl in skylib

Avoid loading versions from @bazel_skylib//:lib.bzl, because it is now
deprecated and is going to be removed in future skylib versions:

https://github.com/bazelbuild/bazel-skylib/blob/master/lib.bzl#L17

Change-Id: I0c5d16a0d32b1a4104dc7af0eeb015df4f37cfb7

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 2c91a4a..47e3966 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -9,7 +9,7 @@
     urls = ["https://github.com/bazelbuild/bazel-skylib/archive/2169ae1c374aab4a09aa90e65efe1a3aad4e279b.tar.gz"],
 )
 
-load("@bazel_skylib//:lib.bzl", "versions")
+load("@bazel_skylib//lib:versions.bzl", "versions")
 
 versions.check(minimum_bazel_version = "0.19.0")