SLING-3243 Update the Slf4j API bundle to 1.7.5

Relaxing the version range for org.slf4j and org.slf4j.spi package to [1.6,1.8).
With this same bundle can be used with slf4j api 1.6.x and 1.7.x

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1541534 13f79535-47bb-0310-9956-ffa450edef68
SLING-3243 Update the Slf4j API bundle to 1.7.5

Updating the version to 1.7.5 for following deps
* slf4j-api
* jcl-over-slf4j
* log4j-over-slf4j

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1541535 13f79535-47bb-0310-9956-ffa450edef68