Update Android Parse Gradle Version (#467)

Gradle dependency is 1.15.8 instead of 1.15.7 since a new release came out

Author: addisonElliott

_includes/android/getting-started.md

@@ -9,7 +9,7 @@ Add dependency to the application level `build.gradle` file.
 
 ```groovy
 dependencies {
-  compile 'com.parse:parse-android:1.15.7'
+  compile 'com.parse:parse-android:1.15.8'
 }
 ```