Scope strong style to .result-info

In 90a8325b0ba3d0cfb91356fe19d81635c313869d I added a declaration for
strong which was global. Although this repo doesn't have govspeak, it
may have in future, so this commit scopes the declaration to the relevent
place.
Merge pull request #40 from alphagov/scope-strong-to-result-info

Scope strong style to .result-info