Factor out Jinja2 dependence.

Suites that do not use Jinja2 don't need Jinja2 to be installed.
Merge pull request #359 from hjoliver/factor-out-jinja2-dependence

Factor out Jinja2 dependence.
Fix gcylc dot view: wasn't plotting non-family tasks in grouped mode.
Merge pull request #361 from hjoliver/fix-gcylc-dot-view-grouping

Fix gcylc dot view: wasn't plotting non-family tasks in grouped mode.