title: Cannot build RabbitMQ plugin because of missing dependencies
body: <p>I would like to start working on my own RabbitMQ plugin, but I cannot build <a href="https://github.com/rabbitmq/rabbitmq-public-umbrella" rel="nofollow noreferrer">https://github.com/rabbitmq/rabbitmq-public-umbrella</a> from source code on clean installation of Ubuntu 16.04. I have installed everything neccessary as described in <a href="https://www.rabbitmq.com/plugin-development.html" rel="nofollow noreferrer">https://www.rabbitmq.com/plugin-development.html</a>. 
  Every time I hit <code>make co</code> I receive following error:
  <code>
  make[2]: Entering directory '/opt/rabbitmq/rabbitmq-public-umbrella/deps/rabbit_common'
   DEP    lager
  tar: /opt/rabbitmq/rabbitmq-public-umbrella/.erlang.mk/hex/lager.tar: Cannot open: No such file or directory
  tar: Error is not recoverable: exiting now
  gzip: stdin: unexpected end of file
  tar: Child returned status 1
  tar: Error is not recoverable: exiting now
  erlang.mk:4710: recipe for target '/opt/rabbitmq/rabbitmq-public-umbrella/deps/lager' failed
  make[2]: *** [/opt/rabbitmq/rabbitmq-public-umbrella/deps/lager] Error 2
  </code></p>
  
  <p>It happens randomly for every dependency which is expected to be in <strong>/.erlang.mk/hex/</strong> directory (lager, ranch, jsx), but this directory is empty (no tar file downloaded).
  The same issue persist when I try to build some existing plugin like <a href="https://github.com/rabbitmq/rabbitmq-auth-backend-http" rel="nofollow noreferrer">https://github.com/rabbitmq/rabbitmq-auth-backend-http</a> on its own.</p>
  
  <p>I am sure I was able to compile the code successfully two days ago and tar files were downloaded to <strong>/.erlang.mk/hex/</strong> when running <strong>make</strong>. I am new to Erlang and all the development tools related to it, so maybe I am missing something.</p>
  
  <p>It's possible that <strong>hex.pm</strong> is not working? I tried everything - restarting the machine, make distclean, I did even create a new VM with Ubuntu on other network but the issue persists.</p>