Take only files ending on .jinja as templates (gh#74).

#74 was caused by swap files of the editor.
`loader.list_templates` takes an extensions argument [1].

Also templates with extensions `html` and `jinja` are supported.

I guess that is why the build failed too because the tests have templates with both extensions.

[1] http://jinja.pocoo.org/docs/api/#jinja2.Environment.list_templates