[BUGFIX beta] Added assert mismatched template compiler version.

See #3728. Using an old version of Ember to precompile templates
can result in an unusual error when trying to render a component
in block form, e.g.

```
  {{#my-component}}Hello{{/my-component}}
```

This commit adds an assert to give the user more information
as to what's going on.
Merge pull request #3729 from machty/block-helper-missing-assert

[BUGFIX beta] Added assert mismatched template compiler version.