fix: header accessibility fixes (#77)

src/views/Header/HeaderBackButton.js

@@ -104,6 +104,7 @@ class HeaderBackButton extends React.PureComponent {
     let button = (
       <TouchableItem
         accessible
+        accessibilityRole="button"
         accessibilityComponentType="button"
         accessibilityLabel={title ? `${title}, back` : 'Go back'}
         accessibilityTraits="button"

src/views/Header/HeaderTitle.js

@@ -9,7 +9,6 @@ const HeaderTitle = ({ style, ...rest }) => (
     {...rest}
     style={[styles.title, style]}
     accessibilityTraits="header"
-    accessible={false}
   />
 );