gitlab_hook.py: fixed "past_tense" function

[cmuller]

The elif action[-2:-1] != "ed" test is always true because the right test should use action[-2:] in the comparison.

This should fix #37.

Tested with:

• past_tense(None) => None
• past_tense('approve') => 'approved'
• past_tense('approved') => 'approved'
• past_tense('actione') => 'actioned'

[sumnerevans]

I'm curious, do you know when this will get deployed to t2bot?