Lock aws-sdk vertion to v1.

The upcoming v2 version of the gem has some API incompatibilities. To avoid problems caused by inadvertently upgrading to v2, Amazon recommends locking the v1 version in Gemfile until any required changes in this app are made to migrate to v2.

See: http://ruby.awsblog.com/post/Tx2OMCYFEZX2I6A/AWS-SDK-for-Ruby-V2-Preview-Release