wot: `revoked on` should displays a humain date rather than a timestamp
```bash
jytou (2pyPs…) from block #547-0000239A13D…
received 3 and sent 6/100 certifications:
| received_expire | received | sent | sent_expire |
|-------------------+---------------------+-------------------+---------------|
| 2018-09-27 | aguy-dev ✔ | aguy-dev | 2018-09-25 |
| 2018-10-13 | matograine-G1Test ✔ | nanocryk | 2018-09-27 |
| 2018-11-16 | esprit ✔ | cgeek | 2018-09-27 |
| | | vit | 2018-09-29 |
| | | kimamila | 2018-10-02 |
| | | matograine-G1Test | 2018-10-13 |
member: False
revoked: True
revoked on: 1539793821
expired: False
wasMember: True
outdistanced: False
```
issue