AWS recently added M4 instances (https://aws.amazon.com/blogs/aws/the-new-m4-instance-type-bonus-price-reduction-on-m3-c4/).

Author: Pradeep Chhetri <pradeep.chhetri89@gmail.com>

Closes #6899 from pradeepchhetri/master and squashes the following commits:

4f4ea79 [Pradeep Chhetri] Added t2.large instance
3d2bb6c [Pradeep Chhetri] Added M4 instances to the list