Update to latest zanata-parent (and slf4j)
Avoid hard-coding http for gravatars

This should fix the warning: "The page at https://zanata... displayed
insecure content from http://www.gravatar.com/avatar/..."
Merge pull request #236 from zanata/slf4j-175

Update to latest zanata-parent (and slf4j)
Merge pull request #237 from zanata/fix-gravatar-warning

Avoid hard-coding http for gravatars