Removed unneeded guzzle dev dependency
Updated dev dependency versions
Moved AbstractAdapter polyfill methods to their own traits.
Added polyfill changes to the changelog.
Moved copy polyfill behaviour to a trait.
Tested Rackspace::readStream
Skip zip copy test for hhvm
Skipped the other copy test for hhvm too.
Added abstract methods for Polyfill that need to be implemented in the consuming class.
Docblock fixes
Moved non-critical (read: hardly used)  Filesystem helpers to plugins.
Scrutinizer Auto-Fixes

This patch was automatically generated as part of the following inspection:
https://scrutinizer-ci.com/g/thephpleague/flysystem/inspections/dd6f5eb6-f719-420b-867e-764cd4ea9438

Enabled analysis tools:
 - PHP Analyzer
 - PHP Change Tracking Analyzer
 - PHP PDepend
Merge pull request #316 from FrenkyNet/scrutinizer-patch-1

Scrutinizer Auto-Fixes
Added ending newline char for StreamCopyTrait
Fixed AdapterInterface return types.
More docblock / return-type fixes
Reduced lots of docblock duplication.
Fixed spec for cache update. Fixed AwsS3 response type for the delete method