Merge tag 'pallet-jclouds-1.5.3' into develop

pallet-jclouds-1.5.3
Updated version for next release cycle
Remove usage of deprecated jclouds method

The ComputeServiceUtils/getSupportedProviders class method is
deprecated and has been removed in jclouds/jclouds@dc4ca1efbd (which
is in the 1.6.0 branch).

This commit changes pallet.compute.jclouds/supported-providers to do
what ComputeServiceUtils/getSupportedProviders and
org.jclouds.rest.providers.Providers/getSupportedProvidersOfType used
to do.

Manual testing verified that for the current classpath, the old and
new implementations of the method both return the same set.
Return nil if p.compute/packager-for-os throws

When looking up the packager for a node, return nil if the underlying
packager-for-os call throws.
Update to pallet 0.8.0-RC.8
Updated project.clj, release notes and readme for 1.5.4
Merge branch 'release/1.5.4'