DOC: Updated 51Degrees git URL to point to a stable version.

The previously documented location doesn't work anymore and must not be
used. Warning for backports, different branches are in use depending on
the version (v3.2.10 for 1.7, v3.2.5 for 1.6).
diff --git a/doc/51Degrees-device-detection.txt b/doc/51Degrees-device-detection.txt
index 1ee912d..71b2eb7 100644
--- a/doc/51Degrees-device-detection.txt
+++ b/doc/51Degrees-device-detection.txt
@@ -14,7 +14,7 @@
 In order to enable 51Degrees download the 51Degrees source code from the
 official github repository :
 
-    git clone https://github.com/51Degrees/Device-Detection
+    git clone https://git.51Degrees.com/Device-Detection.git -b v3.2.10
 
 then run 'make' with USE_51DEGREES and 51DEGREES_SRC set. Both 51DEGREES_INC
 and 51DEGREES_LIB may additionally be used to force specific different paths