fix py3 bug when running `fdroid build` with a buildserver

fix py3 bug when running `fdroid build` with a buildserver

File "/var/lib/jenkins/workspace/fdroidserver-makebuildserver/fdroidserver/build.py", line 1119, in main
    options.onserver, options.refresh):
  File "/var/lib/jenkins/workspace/fdroidserver-makebuildserver/fdroidserver/build.py", line 954, in trybuild
    build_server(app, build, vcs, build_dir, output_dir, force)
  File "/var/lib/jenkins/workspace/fdroidserver-makebuildserver/fdroidserver/build.py", line 403, in build_server
    output += get
TypeError: Can't convert 'bytes' object to str implicitly

closes #164 https://gitlab.com/fdroid/fdroidserver/issues/164