travis-ci: Simplified

- Replaced double by single quotes, because `!` needs otherwise escaping
- Fixed the ` Cannot stat: No such file or directory` warning in travis-ci during the build process

.travis.yml

@@ -23,11 +23,11 @@ before_deploy:
   # they failed to build.
   - go get github.com/mitchellh/gox
   - mkdir releases
-  - gox -osarch="!netbsd/arm !openbsd/386 !openbsd/amd64 !netbsd/386 !netbsd/amd64" -output="releases/{{.Dir}}_`git describe --tags --abbrev=0`_{{.OS}}_{{.Arch}}/{{.Dir}}"
+  - gox -osarch='!netbsd/arm !openbsd/386 !openbsd/amd64 !netbsd/386 !netbsd/amd64' -output="releases/{{.Dir}}_`git describe --tags --abbrev=0`_{{.OS}}_{{.Arch}}/{{.Dir}}"
   # Below tar command will log an error because either ...-hetzner or
   # ...-hetzner.exe are missing. The process continues, though and the
   # created tarballs are fine.
-  - find releases -maxdepth 1 -mindepth 1 -type d -exec tar -cvzf {}.tar.gz -C {} docker-machine-driver-hetzner docker-machine-driver-hetzner.exe \;
+  - find releases -maxdepth 2 -mindepth 2 -type f -exec bash -c 'tar -cvzf "$(dirname {}).tar.gz" -C "$(dirname {})" $(basename {})' \;
 
 deploy:
   # See https://docs.travis-ci.com/user/deployment/releases/