Finish the switch to rubyzip

We switched from `zipruby` to `rubyzip` in 2018
(commit d65d5a3)

However, we forgot to change the `gemspec` in the proces (oopsie), so
apps that use this gem are still installing `zipruby`, which hasn't been
touched in 10 years, and doesn't compile today without magic compiler
flags.

This finishes that upgrade.

I chose version `1.3.0` because our main app happens to be using another
gem which requires `rubyzip`, and it specifies `< 2.0.0`, and I want to
limit the rabbit hole i'm digging for myself here. But ideally we'd move
this to the latest version.

spreadsheetx.gemspec

@@ -43,7 +43,7 @@ Gem::Specification.new do |s|
   s.specification_version = 3
 
   s.add_runtime_dependency('libxml-ruby', ['~> 3.1.0'])
-  s.add_runtime_dependency('zipruby', ['~> 0.3.6'])
+  s.add_runtime_dependency('rubyzip', ['~> 1.3.0'])
   s.add_development_dependency('bundler', ['~> 1.0.0'])
   s.add_development_dependency('jeweler', ['~> 1.6.2'])
   s.add_development_dependency('rcov', ['>= 0'])