Remove reverse-dependency for ghc 7.6's aws-ec2

Currently Transaction's functional dependency has both a->r and r-> a.
I think "a" is request and "r" is response, so r->a is not needed.
In ghc7.6, r->a causes compile-error for aws-ec2(https://github.com/junjihashimoto/aws-ec2).
compile-error is 
https://gist.github.com/junjihashimoto/c4ad857d459e91b20cb5
.