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: Ie722351de5254f611cf3489c8689922c8346af7f
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

WORKSPACE

@@ -9,7 +9,7 @@ http_archive(
     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")