do not specify RN version in gradle

previously this would cause gradle to fetch old rn version even though it wasn't needed

android/build.gradle

@@ -30,6 +30,6 @@ repositories {
 }
 
 dependencies {
-  compile 'com.facebook.react:react-native:0.19.+'
+  compile 'com.facebook.react:react-native:+'
   compile 'com.facebook.conceal:conceal:1.1.3@aar'
 }