pipelines: retain ~/.cache/pip between runs

provision: target newer Nginx versions (>1.9.5 in this case)

provision: use check_mode instead of always_run (deprecated)

provision: fail_timeout is ignored if there's only one upstream server

ui: more jshint enforcing

js: remove trailing spaces

workspaces: use this.$() shortcut instead of this.$el.find()

static: update bundled backbone.shard.js

pipelines: it's too expensive to test provisioning

docs: limit lint status badge to default branch