Throw error if heading size is higher than 6

[sodoku]

Got a quite cryptic error:

Invariant Violation: Element type is invalid: expected a string (for built-in components) or a class/function (for composite components) but got: undefined.
You likely forgot to export your component from the file it's defined in, or you might have mixed up default and named imports. Check the render method of `Heading`.

Took me a while to realize, that I tried to use a Heading with size 7. The proposed error is more obvious to me.

[ryan-roemer]

Hi @sodoku -- Thanks for the PR. Currently this is failing CI -- can you read through our PR submission guidelines + practices to get this PR in shape? https://github.com/FormidableLabs/spectacle/blob/master/CONTRIBUTING.md#before-submitting-a-pr

Thanks!

[sodoku]

@ryan-roemer thanks for telling me so nicely and sorry for wasting extra time. I committed a fix.