wot: `revoked on` should displays a humain date rather than a timestamp

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