Reflect renaming of the plugin to 'avatars-gravatar'

To harmonize plugin project names, the plugin's project got renamed
from 'plugins/avatars/gravatar' (slash before 'gravatar') to
'plugins/avatars-gravatar' (dash before 'gravatar').

We reflect this renaming in the BUCK file, to make the plugin
compilable again.

Change-Id: I5bf0e29b8ddbef708f0dc9b087c2c10faa1b19bc