Fix: platform-specific `builder_settings` bug

Since platform-specific settings are specified under `builder_setting['osx']` in `LaTeXTools.sublime-settings`, `TraditionalBuilder` doesn't resolve custom settings.

```
"builder_settings" : {
(...)
		// Platform-specific settings:
		"osx" : {
			// See README or third-party documentation
		},
(...)
```

Just added `plat` key before `.get` method.