Fixed class methods being mistaken for instance methods

Fixes agronholm#330.

Raise RuntimeError if starting the scheduler under uWSGI without threads

Compare next_date to start_date, not previous_fire_time

Bumped up the fallback version

Added Gitter webhook

Updated tox-travis configuration syntax

Updated tox-travis configuration syntax

Removed erroneous changelog entry

Documented the migration and the scheduler states

Set a minimum version for tzlocal to prevent CentOS related issue rep…

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