Fix blog link

[hugovk]

http://blog.replicated.com/2016/02/05/refactoring-a-dockerfile-for-image-size/

has moved to

https://blog.replicated.com/refactoring-a-dockerfile-for-image-size/

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[hugovk]

Sorry, this is a duplicate of #161.