ec2: bump API version for RunInstances

This is needed to support BlockDeviceMapping.n.Ebs.{VolumeType,Iops,Encrypted}.

I've been through the differences between 2011-12-15 (what was being used) and
2014-10-01 (current AWS API), and I've found the following differences:
  - AddressingType only in old; marked deprecated
  - BlockDeviceMapping.n.Ebs.{VolumeType,Iops,Encrypted} only in new
  - NetworkInterface.n.PrivateIpAddresses.n.{PrivateIpAddress,Primary}
    only in new
  - NetworkInterface.n.{Secondary,Associate}PrivateIpAddressCount only
    in new
  - IamInstanceProfile.Arn only in new

None of the new fields are required, and the defaults match the existing
behaviour.