Remove instructions for kubectl v1.5.x
Remove instructions for kubectl v1.5.x

We only support clusters v1.7.x and above, and kubectl versions should
match their clusters.

Signed-off-by: Lorenzo Manacorda <86995a091f8a1385082bd10715048b52ee386a6a@kinvolk.io>

kinvolk/habitat-operator,kinvolk/habitat-operator