Fix date for Gravatar migration

diff --git a/db/migrate/20131029121300_set_default_gravatar_to_false_for_privacy.rb b/db/migrate/20150818224516_set_default_gravatar_to_false_for_privacy.rb
similarity index 100%
rename from db/migrate/20131029121300_set_default_gravatar_to_false_for_privacy.rb
rename to db/migrate/20150818224516_set_default_gravatar_to_false_for_privacy.rb