Increase minimum AWS version for modify_volume
Increase minimum AWS version for modify_volume

In order to enable updating EBS volumes on the fly, the SDK needs to
support the
[#modify_volume](http://docs.aws.amazon.com/sdkforruby/api/Aws/EC2/Client.html#modify_volume-instance_method).
This has been introduced in version
[2.7.8](https://github.com/aws/aws-sdk-ruby/commit/9e0558fdfb0eec9ed03f500d552edd3b36cdaa9f#diff-be5c280d1f1bc5bd07cf35078448e1caR1668)
of aws-sdk gem. Gem version is bumped to the latest available version
(2.7.8)

ManageIQ/manageiq-providers-amazon,ManageIQ/manageiq-providers-amazon,durandom/manageiq-providers-amazon