Fix native install tasks failing

Previously sence `rake spec` required the `compile` step,
`scripts/test-gem-install` failed because `extconf.rb` was
removed. Fix this by making the default task depend on the `compile`
task, but the `spec` task independent of the `compile` task.

Rakefile

@@ -142,5 +142,4 @@ task "set-version-to-timestamp" do
   puts "NOTE: wrote version as \"#{fake_version}\""
 end
 
-task :spec    => :compile
-task :default => :spec
+task default: [:compile, :spec]