Fix 1.8.25 header whitespace
Force use of minitest 4.x when 5.x is present

Ruby trunk has not been updated to minitest 5.x yet so the tests have
warnings and some errors due to incompatibilities.

Now minitest 4.x is forced through both `rake` and `ruby -Ilib test/...`
Force minitest 4.7 on travis
Update tested versions for 1.8.x on travis