bundler-audit 0.7.0

Use bundler-audit 0.7.0, currently in pre-release. It has an
up-to-date Thor dependency, which fixes an incompatibility with Bundler
2.

Here is a trail of bug reports and fixes against the repo:

* <rubysec/bundler-audit#239>
* <rubysec/bundler-audit#237>
* <rubysec/bundler-audit#252>

lib/suspenders/generators/advisories_generator.rb

@@ -3,7 +3,10 @@
 module Suspenders
   class AdvisoriesGenerator < Generators::Base
     def bundler_audit_gem
-      gem "bundler-audit", require: false, group: %i[development test]
+      gem "bundler-audit",
+          require: false,
+          group: %i[development test],
+          git: "https://github.com/rubysec/bundler-audit.git", branch: "0.7.0"
       Bundler.with_clean_env { run "bundle install" }
     end