vendor.conf: Bump CNI to v0.6.0

Neither the nominal version nor vendored content had changed since
a031b83 (Initial checkin from CRI-O repo, 2017-11-01):

  $ git ls-tree origin/master -- vendor/github.com/containernetworking/cni
  040000 tree 744c091    vendor/github.com/containernetworking/cni
  $ git ls-tree a031b83 -- vendor/github.com/containernetworking/cni
  040000 tree 744c091    vendor/github.com/containernetworking/cni
  $ git grep /cni a031b83 -- vendor.conf
  a031b83:vendor.conf:github.com/containernetworking/cni v0.4.0

I'm not quite sure which upstream version the old vendored content
came from, but it certainly wasn't v0.4.0.  I've bumped our nominal
version to v0.6.0 and re-vendored with:

  $ vndr github.com/containernetworking/cni
  $ git add -A vendor/github.com/containernetworking/cni

to generate this commit.  The only change to the vendored content is
text in the README (unlikely to break anything ;), and sitting on an
upstream tag is nice, so I think this is good enough.

Signed-off-by: W. Trevor King <[email protected]>

Closes: #750
Approved by: mheon

Author: wking

vendor.conf

@@ -8,7 +8,7 @@ github.com/boltdb/bolt master
 github.com/buger/goterm 2f8dfbc7dbbff5dd1d391ed91482c24df243b2d3
 github.com/containerd/cgroups 7a5fdd8330119dc70d850260db8f3594d89d6943
 github.com/containerd/continuity master
-github.com/containernetworking/cni v0.4.0
+github.com/containernetworking/cni v0.6.0
 github.com/containernetworking/plugins master
 github.com/containers/image 88423e35d5f11939b0db4fb8f2939fc04adf2463
 github.com/containers/storage e454acf9874d6c399ccb87d03fc4dcbe8c1a4cc6