Fix --cover-min-percentage with --cover-branches

When using the `--cover-min-percentage` option, the regex responsible
for extracting the cover percentage fails due to extra columns in the
result text when the `--cover-branches` option is also specified.

Using an optional (and non-capturing) group within the regex, we can
recognize a match whether or not the extra columns are present. This
fixes #626.

Note: The two other lines modified in this commit have only had the
trailing whitespace on the line removed.