Make Env.read_env()'s "overrides" argument actually override variables

[fdemmer]

Proposed fix and clarification in docstring for #103. As mentioned in the issue I am not 100% clear on what the intention with overrides was, but as it is called "override" I thought it maybe should override variables ;)

[coveralls]

Coverage remained the same at 88.312% when pulling ee62426 on fdemmer:103-read_env-overrides into 7e50e34 on joke2k:develop.

[coveralls]

Coverage increased (+3.8%) to 89.51% when pulling 31fd174 on fdemmer:103-read_env-overrides into c262002 on joke2k:develop.

[QasimK]

There is a bit of history with regards to variable precedence: e.g. #58 #66 #88. This may be a backwards-incompatible change :(

[sergeyklay]

PR was closed by mistake. Reopened it.

[sergeyklay]

This is implemented in develop branch. Thank you for the pathc, and for helping us make django-environ better. And I am sorry about the delay.