Bumped Polymer version

When running `bower install`, there is a version conflict:

```
Unable to find a suitable version for polymer, please choose one:
    1) polymer#~0.3.4 which resolved to 0.3.5 and is required by seed-element 
    2) polymer#>=0.4.0 <1.0.0 which resolved to 0.4.0 and is required by core-component-page#0.4.0
```

If I choose the `0.3.4` version, I get an error when running the demo:

```js
Uncaught TypeError: undefined is not a function
var declarations = Platform.deliverDeclarations();
```

(this is the same error than https://github.com/Polymer/polymer/issues/391)

Bumping Polymer version to `0.4.0` fixes the issues.