Don't update hijacker/hijacked last login; fixes 5d6e69e for Django ≥2.0

[zopieux]

See #149.

[coveralls]

Coverage increased (+0.2%) to 92.149% when pulling 6f89789 on Zopieux:develop into ac08b69 on arteria:develop.

[zopieux]

😕 My face when the patch works locally but not in Travis. Stay tuned.

[zopieux]

Actually it did not, I can't read. Django 2.0 changed the way it connects the signal we want to hide (uses an UID instead of just the receiver function id). Requires disconnecting both cases, or using version-specific code (I don't want the latest). This update should work and the test covers the hijacker case: we don't want to update the hijacker last login neither!

[coveralls]

Coverage increased (+0.3%) to 92.308% when pulling 0160dbc on Zopieux:develop into ac08b69 on arteria:develop.