[fix] Linter threw JS error when role is undefined (#53)

```
TypeError: roleValue.toLowerCase is not a function
```

Author: christianvuerings

src/rules/img-has-alt.js

@@ -24,7 +24,7 @@ module.exports = context => ({
 
     const roleProp = getAttribute(node.attributes, 'role');
     const roleValue = getAttributeValue(roleProp);
-    const isPresentation = roleProp && roleValue.toLowerCase() === 'presentation';
+    const isPresentation = roleProp && typeof roleValue === 'string' && roleValue.toLowerCase() === 'presentation';
 
     if (isPresentation) {
       return;

tests/src/rules/img-has-alt.js

@@ -129,6 +129,7 @@ ruleTester.run('img-has-alt', rule, {
     { code: '<img alt />;', errors: [ altValueError('img') ], parserOptions },
     { code: '<img alt={undefined} />;', errors: [ altValueError('img') ], parserOptions },
     { code: '<img src="xyz" />', errors: [ missingPropError('img') ], parserOptions },
+    { code: '<img role />', errors: [ missingPropError('img') ], parserOptions },
     { code: '<img {...this.props} />', errors: [ missingPropError('img') ], parserOptions },
     { code: '<img alt={false || false} />', errors: [ altValueError('img') ], parserOptions },