"warning" should not be used anymore

[rugk]

Many examples in the code include "warning" for the severity. As of EsLint 5.3.0 (maybe earlier, don't know) this is not wanted:

Configuration for rule "mocha/valid-suite-description" is invalid: 	Severity should be one of the following: 0 = off, 1 = warn, 2 = error (you passed '"warning"'). 

So replace them with "warn".

[lo1tuma]

Thanks, this should be definitely fixed in our documentation.

[lo1tuma]

Fixed by #181.