make a error reporting by mail ENV vars optional, thus fixing test builds

**Hotfix: do not merge in github.**

To replicate drone.io's troubles with our tests:

First, cd into the root of doaj repo

    git pull
    git checkout hotfix/fix_tests_provide_config_defaults
    mv app.cfg app.cfg.1
    mv dev.cfg dev.cfg.1

OK now you're just like drone.io. Now do:

    DOAJENV=dev nosetests -v doajtest/unit/

If you don't get errors then the hotfix is successful. The idea is that you can now run the test suite by just setting DOAJENV and *nothing* else. Because drone.io does not have the env-specific config files, like dev.cfg.

So if all looks good, do

    mv app.cfg.1 app.cfg
    mv dev.cfg.1 dev.cfg
    git flow hotfix finish fix_tests_provide_config_defaults
    git checkout develop && git push
    git checkout master && git push

P.S. Successful drone build with all our unit tests running! https://drone.io/github.com/DOAJ/doaj/31