fix undefined str variable for html coverage

Order for variables definition is wrong in 3f78f10,
so, html coverage page print `<undefined></undefined>`
because str is undefined in `jade.compile(str, { filename: file });`.

lib/reporters/html-cov.js

@@ -21,9 +21,9 @@ exports = module.exports = HTMLCov;
 function HTMLCov(runner) {
   var jade = require('jade');
   var file = join(__dirname, '/templates/coverage.jade');
+  var str = readFileSync(file, 'utf8');
   var fn = jade.compile(str, { filename: file });
   var self = this;
-  var str = readFileSync(file, 'utf8');
 
   JSONCov.call(this, runner, false);