Removed 'target' from caches, as recommended by https://levans.fr/rust_travis_cache.html. Hopefully this shows significant build speed improvements.

.gitlab-ci.yml

@@ -34,7 +34,3 @@ documentation:
   #    - echo '<meta http-equiv="refresh" content="0; url={{ LIBRARY NAME }}">' > public/index.html
   only:
     - master
-
-cache:
-  paths:
-    - target/