Remove ES2015 let from css block

[timhall]

A let has snuck by the in the generated code for adding css:

Input:

<h1>Hello {{name}}!</h1>

<style>
  h1 {
    color: red;
  }
</style>

Output:

let addedCss = false;
function addCss () {
	var style = createElement( 'style' );
	style.textContent = "\n  h1[svelte-895830737], [svelte-895830737] h1 {\n    color: red;\n  }\n";
	appendNode( style, document.head );

	addedCss = true;
}

function renderMainFragment ( root, component ) {
// ...

It is sneaking by the ES5 syntax check in the tests (introduced in #82) because the css block is appended before the starting point of the check, renderMainFragment. I played with possible ways of updating the check, but I couldn't find a non-hacky approach since there are few types of blocks that may appear (at least applyComputations and addCss).

Thanks so much for your work on this framework, it's great!

[Rich-Harris]

Ha, whoops! Thank you for the fix. Will think about how we might change that test