Fixes touchable formula on iOS

Author: benoitvallon

src/common/components/FormulaeRender.ios.js

@@ -3,16 +3,17 @@
 import React, {
   StyleSheet,
   Text,
-  View
+  View,
+  TouchableHighlight
 } from 'react-native';
 
 export default function (props, state) {
   return (
     <View style={styles.formulae}>
       {this.state.displayFormulae.map(function(formula) {
-        return <View style={getFormulaStyles(formula.operator)}>
+        return <TouchableHighlight style={getFormulaStyles(formula.operator)} onPress={this.handleClick.bind(this, formula)} underlayColor='#cdcdcd'>
           <Text style={styles.text}>{formula.literal}</Text>
-        </View>
+        </TouchableHighlight>
       }, this)}
     </View>
   );