Make steps extension only match at start of lines

The steps extension was matching any text starting `s[0-9]+.` rather
than only matching at the start of a line. This caused an issue with
some HMRC content that contained a sentence ending in a match.
Restrict nokogiri versions

Nokogiri 1.6.x introduces test failures due to whitespace changes.
Merge pull request #21 from alphagov/fix-steps-extensions

Make steps extension only match at start of lines