Specify aws-sdk version to 2.0.22 and up
Specify aws-sdk version to 2.0.22 and up

Fixes https://github.com/travis-ci/dpl/issues/325

Mac image has aws-sdk 1.64.0 pre-installed, so aws-sdk 2.x is not
installed on the fly.

travis-ci/dpl,computology/dpl,travis-ci/dpl,travis-ci/dpl,flowlo/dpl,testfairy/dpl,testfairy/dpl,mattk42/dpl,pengsrc/dpl,sebest/dpl,LoicMahieu/dpl,jsloyer/dpl,testfairy/dpl,Misfit-SW-China/dpl,mathiasrw/dpl,georgantasp/dpl,Zyko0/dpl,jonathan-s/dpl,axelfontaine/dpl,clayreimann/dpl