chore(ec2): introducing c4 instance types

This change adds the recently introduces c4 instance types to the
list of allowed types in the CloudFormation template.

Author: Dan van den Berg

contrib/ec2/deis.template.json

@@ -48,6 +48,11 @@
         "c3.2xlarge",
         "c3.4xlarge",
         "c3.8xlarge",
+        "c4.large",
+        "c4.xlarge",
+        "c4.2xlarge",
+        "c4.4xlarge",
+        "c4.8xlarge",
         "c1.medium",
         "c1.xlarge",
         "cc2.8xlarge",