Fix placeholder documentation
Fix another paragraph
Documentation revision. Thanks @mkoistinen
Merge pull request #2249 from yakky/feature/ph_docs

Fix PlaceholderField documentation
Add --do-migrations / CMS_DO_MIGRATIONS switch to enable / disable running migrations
Use migrations in travis environments
Rename do_migrations option to migrate. Remove environment variable to enable migrations.
Merge remote-tracking branch 'origin/develop' into tests-cleanup