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
diff --git a/BUCK b/BUCK
index c15f524..1606b51 100644
--- a/BUCK
+++ b/BUCK
@@ -1,8 +1,5 @@
 gerrit_plugin(
-  name = 'gravatar',
+  name = 'avatars-gravatar',
   srcs = glob(['src/main/java/**/*.java']),
   resources = glob(['src/main/resources/**/*']),
-  manifest_entries = [
-    'Gerrit-PluginName: gravatar-avatar-provider'
-  ]
 )