travis: remove "apt" cache

"apt" is not a valid option in this file, and caching apt packages
is not recommended according to the Travis documentation:
https://docs.travis-ci.com/user/caching/#things-not-to-cache

> The cache’s purpose is to make installing language-specific dependencies
> easy and fast, so everything related to tools like Bundler, pip, Composer,
> npm, Gradle, Maven, is what should go into the cache.
>
> Large files that are quick to install but slow to download do not benefit
> from caching, as they take as long to download from the cache as from the original source:
>
> - Android SDKs
> - Debian packages
> - JDK packages
> - Compiled binaries
> - Docker images

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

.travis.yml

@@ -5,8 +5,6 @@ go:
   - 1.13.x
   - 1.14.x
 
-cache: apt
-
 go_import_path: gopkg.in/rethinkdb/rethinkdb-go.v6
 
 install: go get -t ./...