Change Mac OS version.
Merge pull request #575 from ximenesuk/dropbox-reqs

Update DropBox docs for develop
merge develop -D org -I include -E exclude: PR 533 (Removing BF anchor until it exists)
merge develop -D org -I include -E exclude: PR 557 (remind that model changes may affect IceMapper)
merge develop -D org -I include -E exclude: PR 572 (Adding contributing docs to OME docs repo readme)
merge develop -D org -I include -E exclude: PR 573 (Continous integration fixes)
merge develop -D org -I include -E exclude: PR 574 (Update web application development page for Django 1.6)