Bump version to 3.20.0 on master

Signed-off-by: Álvaro Brey Vilas <alvaro.brey@nextcloud.com>

build.gradle

@@ -87,7 +87,7 @@ repositories {
 
 // semantic versioning for version code
 def versionMajor = 3
-def versionMinor = 19
+def versionMinor = 20
 def versionPatch = 0
 def versionBuild = 0 // 0-50=Alpha / 51-98=RC / 90-99=stable