Bumped manifest to 4.507

<?xml version="1.0" encoding="utf-8"?>

<manifest

    xmlns:android="http://schemas.android.com/apk/res/android"

    android:versionCode="18006"

    android:versionName="4.506" package="com.fsck.k9"

    android:versionCode="18007"

    android:versionName="4.507" package="com.fsck.k9"

    >

    <uses-sdk

       android:minSdkVersion="8"