Merge pull request react-bootstrap#1545 from carlyle/a11y-fix-alert-close

a11y: fixing the tab-index for the non-screenreader version of the Alert dismiss button

Author: jquense

src/Alert.js

@@ -23,7 +23,8 @@ let Alert = React.createClass({
         type="button"
         className="close"
         onClick={this.props.onDismiss}
-        aria-hidden="true">
+        aria-hidden="true"
+        tabIndex="-1">
         <span>&times;</span>
       </button>
     );