Use the react-native version of the project (#84)

Instead of specifying a RN version ourselves, we should inherit it from the project.
This fixes building on the latest version of RN.

Author: nielsdB97

android/build.gradle

@@ -16,5 +16,5 @@ android {
 }
 
 dependencies {
-    compile 'com.facebook.react:react-native:0.20.+'
+    implementation "com.facebook.react:react-native:+" // From node_modules
 }