let's use latest version of SLF4j API

See #108, release log:

• 4b4d092 by @yegor256: #108 rultor
• af79870 by @yegor256: #108 all clean
• 9befa43 by @yegor256: #108 year up to 2020
• 7f9695f by @yegor256: #108 qulice polished
• ef67f7a by @yegor256: #108 headers fixed
• c5a9cba by @yegor256: #108 from junit4 to juni5
• 01b1d41 by @yegor256: openjdk8 instead of oracle
• a581764 by @yegor256: HTTPS for jpeek

Released by Rultor 1.69.1, see build log