Fix sorting issue with line number nil

A sorting exception occurs when sorting two lints on one file where one
lint has line number 'nil' and the other one not. This commit fixes the
issue by treating 'nil' line numbers as 0.

Change-Id: I65ebc328f516f5e5eae1d238e5cf959a24cfb089
Reviewed-on: http://gerrit.causes.com/40302
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Fix warning message in DefaultReporter spec

We were seeing the following warning message in the `DefaultReporter`
spec:

    `raise_error` was called with non-proc object nil

The solution was to change the use of `should_not` to use an expectation
instead.

Change-Id: Ia1758ac8c6c64087807c153d270487e16c001093
Reviewed-on: http://gerrit.causes.com/40303
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>