Upgrade to slf4j 1.7.18

26th of February, 2016 - Release of SLF4J 1.7.18

Initialization of the slf4j-simple module is now thread safe fixing SLF4J-356.

Fixed Import-Package statement in MANIFEST.MF for the slf4j-log4j12 module. This isssue was reported by Mathieu Sebire in SLF4J-357 who also provided the relevant pull request.
19th of February, 2016 - Release of SLF4J 1.7.17

When running under Android, skip binding ambiguity check during initialization for better performance. The fix introduced in version 1.7.14 was not effective.