[logback-dev] [qos-ch/logback] ecf38b: use slf4j-api version 1.7.21

[Ceki Gulcu]

  Branch: refs/heads/master
  Home:   https://github.com/qos-ch/logback
  Commit: ecf38b32bb45b71e8babc853c40560d75ef8e259
      https://github.com/qos-ch/logback/commit/ecf38b32bb45b71e8babc853c40560d75ef8e259
  Author: Ceki Gulcu <ceki at qos.ch>
  Date:   2016-12-09 (Fri, 09 Dec 2016)

  Changed paths:
    M logback-bom/pom.xml
    M pom.xml

  Log Message:
  -----------
  use slf4j-api version 1.7.21