Ensure lints always have line number

In the unlikely case where a linter would return a lint without a line
number, we want to gracefully handle the situation.

Rather than have each reporter deal with this themselves, simply assign
a default line number of 0 in the case where no line number is provided.

Bugs where 0s are reported for line numbers can then be filed and
addressed accordingly, without `haml-lint` crashing.

Change-Id: Ia697f53f2265d1ac71b161392dfcee5d886147f2
Reviewed-on: http://gerrit.causes.com/42343
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>