Various cleanup

@jerryk55 Please review.

~~I was also going to add a rescue around the `require 'ffi-vix_disk_lib'` in spec_helper.rb with a note that the user has to install the VixDiskLib in order to run the specs, and it can't be redistributed by the gem itself.~~
EDIT: I added it.

On that note, can we even run any specs on Travis?  Can we install the VixDiskLib automatically on Travis? /cc @roliveri 
EDIT: Spoke to @roliveri and this might not be possible.  A few possibilities in the future would be
- Tweak the ffi_library call when running specs to allow it to not load a library, then use something like camcorder for testing
- Create a "fake" VDDK installation that can be built and installed by Travis, and which provides the external declarations needed by the FFI calls, then use camcorder.