ci: update deployment for doc rearrangement

This fixes CI to handle the new documentation files. We also continue to
do more cleanup. In particular, we devise a nicer way of detecting the
most recent Cargo OUT_DIR by writing a dummy file, and looking for the
most recently modified version of that file.

.travis.yml

@@ -73,7 +73,7 @@ before_deploy: ci/before_deploy.sh
 deploy:
   provider: releases
   file_glob: true
-  file: deployment/${PROJECT_NAME}-${TRAVIS_TAG}-${TARGET}.*
+  file: deployment/${PROJECT_NAME}-${TRAVIS_TAG}-${TARGET}.tar.gz
   skip_cleanup: true
   on:
     condition: $TRAVIS_RUST_VERSION = nightly