https://jcenter.bintray.com/org/slf4j/slf4j-simple/maven-metadata.xml
Currently contains the following lines:

  <version>2.0.0-alpha1</version>
  <versioning>
    <latest>2.0.0-alpha1</latest>
    <release>2.0.0-alpha1</release>

But, if I am not misunderstanding maven versioning, it should actually be like this, so that the stable version is the default:

  <version>1.7.30</version>
  <versioning>
    <latest>2.0.0-alpha1</latest>
    <release>1.7.30</release>

I have so far only looked into slf4j-simple. but this might affect other dependencies as well.