Fixed recent bug in task.cylc_env

The task class cylc_env dict was being modified slightly by each task
instance. Each task re-alters it appropriately, but some a chance of
race condition type error?
Finish it.
Merge pull request #406 from hjoliver/fix-task-cylc-env-bug

Fixed recent bug in task.cylc_env
Merge remote branch 'upstream/master' into 204.suite-env-file