Upgrade to Log4j 2.9.1

[jasontedor]

This commit upgrades the Log4j dependency, picking up a fix for an issue with handling stack traces on JDK 9.

Relates apache/logging-log4j2#109, relates #26464, relates #26467

[jasontedor]

Since our PR builds do not test on JDK 9:

07:34:22 ⌂61% [jason:~/src/elastic/elasticsearch] log4j-2.9.1+ ± JAVA_HOME=`/usr/libexec/java_home -v 9` gradle :qa:evil-tests:test -Dtests.class=org.elasticsearch.common.logging.EvilLoggerTests
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Elasticsearch Build Hamster says Hello!
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  Gradle Version        : 4.1
  OS Info               : Mac OS X 10.12.6 (x86_64)
  JDK Version           : Oracle Corporation 9 [Java HotSpot(TM) 64-Bit Server VM 9+181]
  JAVA_HOME             : /Library/Java/JavaVirtualMachines/jdk-9.jdk/Contents/Home
  Random Testing Seed   : 70A4BA63301F831E
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> Task :qa:evil-tests:test
Slow Tests Summary:
 15.44s | org.elasticsearch.common.logging.EvilLoggerTests

==> Test Summary: 1 suite, 9 tests
   [junit4] JVM J0:     0.62 ..    16.76 =    16.14s
   [junit4] Execution time total: 16 seconds
   [junit4] Tests summary: 1 suite, 9 tests


BUILD SUCCESSFUL in 1m 41s
15 actionable tasks: 10 executed, 5 up-to-date
07:36:06 ⌂83% [jason:~/src/elastic/elasticsearch] log4j-2.9.1+ ± 

[nik9000]

Thanks for fixing this upstream! You are such a good citizen.