Fix 'context is value' bug

Hi there,

This pull request fixes a crash that occurs when options.context is absent or not a function.

E.g. this config causes the following crash:

```
var htmlPages = compileHandlebars('pages', ['*.hbs'], {
    partials: 'pages/partials'
});

Crashes at:
TypeError: Property 'context' of object [object Object] is not a function
  at /vagrant/node_modules/broccoli-handlebars/index.js:79:35
  at Array.map (native)
  at /vagrant/node_modules/broccoli-handlebars/index.js:69:33
```

The crash happens because - even if options.context is not a function - it is still called as one on this line: https://github.com/moudy/broccoli-handlebars/blob/master/index.js#L79

I tested this by rerunning the code post-fix, and no crash occurs.

Cheers

Greg