Bump version number to 0.9.3.
Handle missing values in output ports.

When Taverna Server marks an output value as "absent" we need to handle this.
The absent value is substituted for an empty value or an empty list as
appropriate.
Update change-log.
Merge commit '0.9.3' into rh/2.4.2

Forward-port the absent output port values fix to the 1.0.0 branch.

* commit '0.9.3':
  Update change-log.
  Handle missing values in output ports.
  Bump version number to 0.9.3.

Conflicts:
	lib/t2-server/port.rb
	t2-server.gemspec
	version.yml
Add a test for the missing output values bug workaround.