resolve deprecation notice about Fixnum

Fixnum was deprecated in Ruby 2.4, so this should be checking for
Integer.

Changed this check to `kind_of?` for backwards compatibility. Fixnum is
a subclass of Integer, even in Ruby < 2.4. `kind_of?` will return true
if the class given is the class of the object, or if it is one of the
superclasses of the object or modules included in the object.
`instance_of?` only returns true for an exact match on the class, does
not take subclassing into consideration.

Signed-off-by: Robb Kidd <robb@thekidds.org>

templates/Vagrantfile.erb

@@ -181,7 +181,7 @@ Vagrant.configure("2") do |c|
         <% options = [] %>
         <% item.each do |storage_option_key, storage_option_value|
              options << "\"--#{storage_option_key}\""
-             if storage_option_value.instance_of? Fixnum
+             if storage_option_value.kind_of? Integer
                options << storage_option_value
              else
                options << "\"#{storage_option_value}\""