Bump Spark version in README to 1.2.1
Bump Spark version in README to 1.2.1

timothymschier/geotrellis-ec2-cluster,timothymschier/geotrellis-ec2-cluster,geotrellis/geotrellis-ec2-cluster,timothymschier/geotrellis-ec2-cluster,lossyrob/geotrellis-ec2-cluster,geotrellis/geotrellis-ec2-cluster,timothymschier/geotrellis-ec2-cluster,lossyrob/geotrellis-ec2-cluster,lossyrob/geotrellis-ec2-cluster,geotrellis/geotrellis-ec2-cluster,geotrellis/geotrellis-ec2-cluster,lossyrob/geotrellis-ec2-cluster