change submodule url to GitHub
update submodules to latest commit

$ git submodule foreach git pull origin master
rename custom_field_aggregator to redmine_custom_field_aggregator

$ mv plugins/custom_field_aggregator /tmp
$ git rm plugins/custom_field_aggregator
$ git submodule add -f https://github.com/occ-corp/redmine_custom_field_aggregator.git \
  plugins/redmine_custom_field_aggregator
Merge pull request #2 from occ-corp/update_submodule