Revert "Merge pull request #147 from mockdeep/simplecov"

This reverts commit 2b91e03fdf382a61cfac3c59d4c57f5dc41de836, reversing
changes made to e587bdc6710dfcd457ea439ef14bdf22a93ba8b6.
Change all require RMagick to rmagick

Post #157, some were introduced by reverting #147
Remove all require_relatives
Add SimpleCov
Fix hard-coded ruby path
Simplify minimum required ImageMagick version checking
Fix test load paths for 1.8 vs. >= 1.9
Merge pull request #153 from gemhome/18_compat

1.8 compatibility changes