switched to Postgres for Heroku

Conflicts:
	Gemfile
	Gemfile.lock
added secret token for Heroku staging environment
added uglifier for Heroku staging environment

not entirely sure why this is required though

Conflicts:
	Gemfile.lock
tweaked configuration for Heroku staging environment

cf. https://devcenter.heroku.com/articles/rails-asset-pipeline
Merge remote-tracking branch 'upstream/skosxl' into inflectionals

Conflicts:
	Gemfile.lock
added empty db directory

without this, creating schema.rb will fail
Merge remote-tracking branch 'upstream/skosxl' into inflectionals
switched to pre-release version of iqvoc_skosxl

cf. https://github.com/innoq/iqvoc_skosxl/compare/fdbbf8c^...6b67143
Merge remote-tracking branch 'upstream/skosxl' into inflectionals

Conflicts:
	Gemfile