Lowercase MAJOR, MINOR, PATCH, MANUAL to reflect reality

All-caps strings reflect something the user should substitute (as in: knife cookbook upload COOKBOOK). The proper options for bump are not to be used capitalized as indicated in this README, and do not stand for "replace this with your string", so should be lowercase.
Merge pull request #89 from jblaine/patch-1

Whoops, didn't mean to close...Lowercase MAJOR, MINOR, PATCH, MANUAL to reflect reality