Fix #148; More robust feature detection

PR #125 added the string Magick::Magick_features

Issue #148 reported compilation failures on
  Mac OSX Yosemite
  Rails 4.2.0.beta4
  rake, version 10.4.0
  ImageMagick 6.8.9-8 Q16 x86_64 (via macports)

in rmmain.c:1724:28:
error: use of undeclared identifier 'MagickSupport'

This PR now makes the flow to use if defined:
GetMagickFeatures() or
  MagickFeatures or
  MagickSupport or
  "unknown"
Merge pull request #156 from gemhome/fix_features_detection

Fix #148; More robust feature detection
Revert "Merge pull request #147 from mockdeep/simplecov"

This reverts commit 2b91e03fdf382a61cfac3c59d4c57f5dc41de836, reversing
changes made to e587bdc6710dfcd457ea439ef14bdf22a93ba8b6.
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