larger build matrix on AppVeyor

new versions of Ruby (with DevKit) have been installed on AppVeyor VM's:
  - 1.9.3 x86 (C:\Ruby193 - default in PATH)
  - 2.0.0 x86 (C:\Ruby200)
  - 2.0.0 x64 (C:\Ruby200-x64)

this updates to run tests on those.

Note: the 2.0.0 versions don't have bundler installed by default, so we need to do that now; this may change in the future.