Teach invalid-requires to better handle comments

When converting a require block like:

```js
var foo = require('foo'),
  // A comment
  bar = require('bar');
```

this transform was producing the unusual output:

```js
var foo = require('foo');

var // A comment
bar = require('bar');
```

This commit fixes this by moving the comment from the require statement
declaration up to the variable declaration.

.eslintignore

@@ -1,2 +1 @@
 node_modules
-test/invalid-requires-test.js

test/invalid-requires-test.js

@@ -1,2 +1,4 @@
+/* eslint-disable one-var */
 var foo = require('foo'),
+  // A comment
   bar = require('bar');

test/invalid-requires-test.output.js

@@ -1,2 +1,5 @@
+/* eslint-disable one-var */
 var foo = require('foo');
+
+// A comment
 var bar = require('bar');

transforms/invalid-requires.js

@@ -20,6 +20,9 @@ module.exports = function(file, api) {
 
         if (i == 0) {
           variableDeclaration.comments = requireStatement.value.comments;
+        } else if (declaration.comments) {
+          variableDeclaration.comments = declaration.comments;
+          declaration.comments = null;
         }
 
         return variableDeclaration;