fix(Android): Make content of buildscript optional (#245)

Fix gradle build error where it can't find specified version of gradle.

android/build.gradle

@@ -38,14 +38,16 @@ android {
 }
 
 buildscript {
-    repositories {
-        jcenter()
-    }
-    dependencies {
-        classpath 'com.android.tools.build:gradle:1.5.0'
+    if (project == rootProject) {
+        repositories {
+            jcenter()
+        }
+        dependencies {
+            classpath 'com.android.tools.build:gradle:1.5.0'
 
-        // NOTE: Do not place your application dependencies here; they belong
-        // in the individual module build.gradle files
+            // NOTE: Do not place your application dependencies here; they belong
+            // in the individual module build.gradle files
+        }
     }
 }