Merge pull request #720 from boegel/v1.8.x

retrying is futile when None is returned by pbs_submit
bump version to 1.8.1, update release notes
Merge pull request #721 from boegel/v1.8.1

bump version to 1.8.1, update release notes
Merge branch 'v1.8.x' into develop

Conflicts:
	easybuild/tools/version.py