BOOKKEEPER-1059: Upgrade to SLF4J-1.7.25
BOOKKEEPER-1059: Upgrade to SLF4J-1.7.25

Author: Matteo Merli <mmerli@apache.org>

Reviewers: Enrico Olivelli <None>

Closes #147 from merlimat/slf4j-1.7

ivankelly/bookkeeper,sijie/bookkeeper,sijie/bookkeeper,ivankelly/bookkeeper,sijie/bookkeeper,sijie/bookkeeper,apache/bookkeeper,apache/bookkeeper,apache/bookkeeper,sijie/bookkeeper,sijie/bookkeeper,apache/bookkeeper,apache/bookkeeper,apache/bookkeeper,sijie/bookkeeper,sijie/bookkeeper,ivankelly/bookkeeper