Fetch released migrations via the GitHub API

This reverts the released migrations to using the GitHub API as was done
in 9fba1a4.  The concern for the GitHub
rate limits in 9568c7a is no longer a
problem now that migrations are in their own repo which should have much
less churn, unlike the main manageiq repo where the migrations would run
on *every* PR.

Fixes ManageIQ#6

lib/tasks_private/spec.rake

@@ -30,22 +30,14 @@ class SetupReleasedMigrations
   private
 
   def released_migrations
-    return [] unless system(fetch_command)
-    files = `git ls-tree -r --name-only #{TEST_BRANCH} db/migrate/`
-    return [] unless $?.success?
+    json = Net::HTTP.get(URI("https://api.github.com/repos/ManageIQ/manageiq/contents/db/migrate?ref=#{RELEASED_BRANCH}"))
 
-    migrations = files.split.map do |path|
-      filename = path.split("/")[-1]
+    migrations = JSON.parse(json).map do |h|
+      filename = h["path"].split("/")[-1]
       filename.split('_')[0]
-    end
+    end.sort
 
     # eliminate any non-timestamp entries
     migrations.keep_if { |timestamp| timestamp =~ /\d+/ }
-  ensure
-    `git branch -D #{TEST_BRANCH}`
-  end
-
-  def fetch_command
-    "git fetch #{'--depth=1 ' if ENV['CI']}http://github.com/ManageIQ/manageiq.git refs/heads/#{RELEASED_BRANCH}:#{TEST_BRANCH}"
   end
 end