Add OpenGL 2.0 requirement

I just got an error report by a user with this exception:

java.lang.UnsupportedOperationException
at android.view.GLES20Canvas.clipPath(GLES20Canvas.java:408)
at com.r0adkll.postoffice.widgets.RippleView.onDraw(SourceFile:220)
at android.view.View.draw(View.java:11281)
at android.view.View.getDisplayList(View.java:10720)
at android.view.ViewGroup.drawChild(ViewGroup.java:5783)
at android.view.ViewGroup.dispatchDraw(ViewGroup.java:5098)
at android.view.View.getDisplayList(View.java:10718)
at android.view.ViewGroup.drawChild(ViewGroup.java:5783)
at android.view.ViewGroup.dispatchDraw(ViewGroup.java:5098)
at android.view.View.draw(View.java:11284)
at android.view.View.getDisplayList(View.java:10720)
at android.view.ViewGroup.drawChild(ViewGroup.java:5783)
at android.view.ViewGroup.dispatchDraw(ViewGroup.java:5098)
at android.view.View.getDisplayList(View.java:10718)
at android.view.ViewGroup.drawChild(ViewGroup.java:5783)
at android.view.ViewGroup.dispatchDraw(ViewGroup.java:5098)
at android.view.View.getDisplayList(View.java:10718)
at android.view.ViewGroup.drawChild(ViewGroup.java:5783)
at android.view.ViewGroup.dispatchDraw(ViewGroup.java:5098)
at android.view.View.draw(View.java:11284)
at android.widget.FrameLayout.draw(FrameLayout.java:450)
at com.android.internal.policy.impl.PhoneWindow$DecorView.draw(PhoneWindow.java:2306)
at android.view.View.getDisplayList(View.java:10720)
at android.view.HardwareRenderer$GlRenderer.draw(HardwareRenderer.java:842)
at android.view.ViewRootImpl.draw(ViewRootImpl.java:2145)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1869)
at android.view.ViewRootImpl.handleMessage(ViewRootImpl.java:2681)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4517)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:995)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:762)
at dalvik.system.NativeStart.main(Native Method)

It's clearly coming from PostOffice using some OpenGL 2.0 API call. 

I find it best to put this requirement into the library from where it originates instead of in my App using the library. The Manifest merger should add it to my app manifest then.

What do you think?

Best regards,
Kash
Merge pull request #12 from kashban/patch-1

Add OpenGL 2.0 requirement