add some prereqs to ensure old Perls get them all

might not reasonable for strict (in core since 5), but just for completeness.
warnings came with 5.6.0, Exporter with 5, base with 5.00405 ...
add missing } in pure perl of looks_like_number
add missing test prereq for $] < 5.008
summarize Changes of 0.27 & add date of 0.26
bump to 0.28