Move nightly builds to Travis CI

This updates previous work in #739 to build
Debian packages and zip files for other OS.

All the build artifacts are uploaded to S3
public bucket `aptly-nightly` so that there's
archive for all the builds.

All `.deb` packages are automatically uploaded
to repo.aptly.info/nightly on build.

aptly-api.service

@@ -6,7 +6,7 @@ Documentation=https://www.aptly.info/doc/api/
 
 [Service]
 Type=simple
-ExecStart=/usr/bin/aptly serve api -no-lock -listen=127.0.0.1:8081
+ExecStart=/usr/bin/aptly api serve -no-lock -listen=127.0.0.1:8081
 
 [Install]
 WantedBy=multi-user.target