phpstan: ignore unmatched ignored errors

[webignition]

Fixes #246

phpstan is configured to ignore some errors. Of these, some are only present under PHP 7.1 or 7.2 and are not present under PHP 7.3.

By default phpstan will return a non-zero return code if an ignored error pattern is not matched. This is currently the case when invoking phpstan on travis under PHP 7.3 and is causing all current PR builds to fail.

This change adds reportUnmatchedIgnoredErrors: false to the phpstan configuration such that phpstan will be silent about unmatched ignored errors.

[webignition]

Build failing currently due to Travis installing Chrome 78 and the version of ChromeDriver supplied with Panther does not appear to support this version of Chrome.

See #245 for details of that issue.

We will need a single PR that addresses both #245 and #246 at the same time to get travis builds passing.

[webignition]

Should close this PR if #248 passes.