update exoplayer and gradle wrapper

* exoplayer 2.13.2 -> 2.13.3
* gradle 6.7.1 -> 7.0
* remove jcenter repository (fixes #29)

gradlew

@@ -130,7 +130,7 @@ fi
 if [ "$cygwin" = "true" -o "$msys" = "true" ] ; then
     APP_HOME=`cygpath --path --mixed "$APP_HOME"`
     CLASSPATH=`cygpath --path --mixed "$CLASSPATH"`
-    
+
     JAVACMD=`cygpath --unix "$JAVACMD"`
 
     # We build the pattern for arguments to be converted via cygpath