Pass parent environment into all child processes

[kdmukai]

Explicitly copies the initial environment from the Cluster into the Sentinel and all worker processes.

Fixes #105

When deployed on AWS Elastic Beanstalk under supervisord the cluster's child worker processes did not have the same environment variables that were originally injected in the supervisord.conf. As a result the workers (presumably) didn't have access to DJANGO_SETTINGS_MODULE and couldn't locate the target task function, causing No module named foo.tasks errors.

Explicitly passing the parent environment into each child process resolved this issue.

Solution based on:
http://stackoverflow.com/a/24664312

[codecov-io]

Current coverage is 90.22%

Merging #106 into master will decrease coverage by -6.28% as of 157907f

@@            master    #106   diff @@
======================================
  Files           39      39       
  Stmts         2664    2671     +7
  Branches         0       0       
  Methods          0       0       
======================================
- Hit           2571    2410   -161
  Partial          0       0       
- Missed          93     261   +168

Review entire Coverage Diff as of 157907f

Powered by Codecov. Updated on successful CI builds.