[BUGFIX] Fix first-child and last-child selectors

[oliverklee]

Fixes #51
Fixes #192

[jjriv]

-The following selectors are implemented, but currently are broken:
-
  * first-child (currently broken)
  * last-child (currently broken)

I'm confused. Are these selectors still broken or not? We are removing the comment saying they are broken, and the PR says this bug is getting fixed, but inside the parens it still says "currently broken".

[jjriv]

-            $containedHtml,
-            $this->subject->emogrify()
+            $htmlRegularExpression,
+            $result
         );```

According to the docs, $result should be a string:
https://phpunit.de/manual/current/en/appendixes.assertions.html#appendixes.assertions.assertRegExp

But I don't see where $result is getting assigned any value before being passed to assertRegExp()

[oliverklee]

I've just removed the two "(currently broken)" from the docs. I'll look into the $result thing now.

[oliverklee]

$result is string because it gets assigned the return value of $this->subject->emogrify().

And I put it into a local variable to separate the execution from the verification according to the four-phase test concept: https://robots.thoughtbot.com/four-phase-test