switch to our fork of rtt

The goal is not to "fork" per-se, but
orocos-toolchain/rtt#300 makes it
impossible to use it (and I don't have the time to track it
without the help of the Intermodalics guys who wrote it in
the first place).

Since I now get some to track and fix some bugs ... Fork
it is :(

source.yml

@@ -99,7 +99,7 @@ overrides:
     #
     # See https://github.com/orocos-toolchain/rtt/pull/302
     - rtt:
-      commit: baaea5022bc79f94770a962e2e16cc0cb28cde0b
+      github: rock-core/rtt
     - (ocl|log4cpp):
       branch: master