increment version number to 0.20.21

NEWS

+ver 0.20.21 (not yet released)
+
 ver 0.20.20 (2018/05/22)
 * protocol
   - fix "modified-since" filter regression

android/AndroidManifest.xml

@@ -2,8 +2,8 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
           package="org.musicpd"
           android:installLocation="auto"
-          android:versionCode="19"
-          android:versionName="0.20.20">
+          android:versionCode="20"
+          android:versionName="0.20.21">
 
   <uses-sdk android:minSdkVersion="14" android:targetSdkVersion="17"/>
 

configure.ac

 AC_PREREQ(2.60)
 
-AC_INIT(mpd, 0.20.20, musicpd-dev-team@lists.sourceforge.net)
+AC_INIT(mpd, 0.20.21, musicpd-dev-team@lists.sourceforge.net)
 
 VERSION_MAJOR=0
 VERSION_MINOR=20
-VERSION_REVISION=20
+VERSION_REVISION=21
 VERSION_EXTRA=0
 
 AC_CONFIG_SRCDIR([src/Main.cxx])