Fix potential memory leak with not releasing attacher, thanks to @k3b

Author: Jawnnypoo

build.gradle

@@ -4,7 +4,7 @@ buildscript {
     }
 
     dependencies {
-        classpath 'com.android.tools.build:gradle:2.1.0'
+        classpath 'com.android.tools.build:gradle:2.1.2'
         classpath 'com.github.dcendents:android-maven-gradle-plugin:1.3'
     }
 }

library/src/main/java/uk/co/senab/photoview/PhotoView.java

@@ -255,6 +255,7 @@ public void setOnSingleFlingListener(PhotoViewAttacher.OnSingleFlingListener onS
     @Override
     protected void onDetachedFromWindow() {
         mAttacher.cleanup();
+        mAttacher = null;
         super.onDetachedFromWindow();
     }