navTemplate should be processed by grunt.template!

angular-ui/bootstrap's nav template includes a reference to the package.json version in a lo-dash template, and I see this as being a fairly common thing to do, whether the version information is coming from bower.json or whatever.

A simple fix for this is to just
```js
options.navContent = grunt.template.process(grunt.file.read(options.navTemplate));
```
rather than simply

```js
options.navContent = grunt.file.read(options.navTemplate);
```

I'll send a patch real quick. But this seems like something that would be sort of an expected behaviour