Deal with regex match groups in excerpt

[Original implementation](https://github.com/rails/rails/pull/15820) has bugs if the regex contains a match group.

**Example:**

    excerpt('This is a beautiful? morning', /\b(beau\w*)\b/i, :radius => 5)
    Expected: "...is a beautiful? mor..."
    Actual: "...is a beautifulbeaut..."

The original phrase was being converted to a regex and returning the text
either side of the phrase as expected:

    'This is a beautiful? morning'.split(/beautiful/i, 2)
    # => ["This is a ", "? morning"]

When we have a match with groups the match is returned in the array.

Quoting the ruby docs: "If pattern is a Regexp, str is divided where the
pattern matches. [...] If pattern contains groups, the respective matches will
be returned in the array as well."

    'This is a beautiful? morning'.split(/\b(beau\w*)\b/iu, 2)
    # => ["This is a ", "beautiful", "? morning"]

If we assume we want to split on the first match – this fix makes that
assumption – we can pass the already assigned `phrase` variable as the place
to split (because we already know that a match exists from line 168).

Originally spotted by Louise Crow (@crowbot) at
https://github.com/mysociety/alaveteli/pull/1557