make gem dependencies reasonably optimistic again (fixes #253)

This reverts the vagrant-libvirt.gemspec changes in commit
19cd8be193fdeb0a8e1b004705ef3d4465c02812.
remove pry debug
Merge pull request #255 from aspiers/remove-debug

remove pry debug
Merge pull request #254 from aspiers/loosen-deps

make gem dependencies reasonably optimistic again (fixes #253)
don't monkey-patch #default_provider

This monkey-patching of Vagrant::Environment#default_provider breaks
with Vagrant 1.7.0, since the method's signature changed.  In any case
Vagrant should do the right thing without "help" from vagrant-libvirt.
allow newer ruby-libvirt (#158)

As per #158, vagrant-libvirt depends on ruby-libvirt transitively
through fog.  Since fog/fog#2706 was resolved, fog now requires
ruby-libvirt ~> 0.5.0, therefore vagrant-libvirt should allow both 0.4.x
and 0.5.x.  Furthermore, according to semantic versioning, we currently
have no good reason to reject anything newer below 1.x.  If such a
restriction is required in the future (e.g. < 0.7.0) then it is likely
that fog's .gemspec will take care of that on our behalf anyway.

Closes #158.
allow cache mode to be set for additional disks
Merge pull request #261 from aspiers/remove-default-provider

don't monkey-patch #default_provider
Merge pull request #262 from aspiers/allow-newer-ruby-libvirt

allow newer ruby-libvirt (#158)
Merge pull request #263 from aspiers/extra-storage-cache-mode

allow cache mode to be set for additional disks
allow preexisting disk volumes via new option