Fixes #17.

Fixes #15.

Work around a bug in GraceDB where normalization of floating-point
GPS times to fixed-precision decimal representation is applied to
JSON-encoded requests but not form-encoded requests. This bug
caused superevent API requests with GPS times specified with more
than 6 decimal places to fail.

See https://git.ligo.org/lscsoft/gracedb/issues/195.

Do not run more than one unit test job at once, because GraceDB
has race conditions when multiple clients are creating superevents.

Fix an argument parsing bug: `client.superevents.update()` failed to
treat the keyword argument `preferred_event=None` the same as omission
of the keyword argument.

Fixes #13.