For people passing by now, in 2017, the new best way to achieve what you want is by using ConstraintLayout like this:
<ImageView
android:layout_width="0dp"
android:layout_height="0dp"
android:scaleType="centerCrop"
app:layout_constraintDimensionRatio="1:1" />
And don't forget to add constraints to all of the four directions as needed by your layout.
Build a Responsive UI with ConstraintLayout
Furthermore, by now, PercentRelativeLayout has been deprecated (see Android documentation).