Travis: build all possible pages for testing
Travis: build all possible pages for testing

When test building the site, build future pages, drafts, and unpublished
pages so that the links in those documents can be checked by
htmlproofer.

bitcoin-core/bitcoincore.org,bitcoin-core/bitcoincore.org,bitcoin-core/bitcoincore.org,bitcoin-core/bitcoincore.org,bitcoin-core/bitcoincore.org,bitcoin-core/bitcoincore.org