Changed subproject URLs to read mode

.gitmodules

 [submodule "conv-ttl-blo-gw"]
 	path = conv-ttl-blo-gw
-	url = git@ohwr.org:level-conversion/conv-ttl-blo/conv-ttl-blo-gw.git
+	url = git://ohwr.org:level-conversion/conv-ttl-blo/conv-ttl-blo-gw.git
 [submodule "conv-ttl-blo-tst"]
 	path = conv-ttl-blo-tst
-	url = git@ohwr.org:level-conversion/conv-ttl-blo/conv-ttl-blo-tst.git
+	url = git://ohwr.org:level-conversion/conv-ttl-blo/conv-ttl-blo-tst.git
 [submodule "conv-ttl-blo-hw"]
 	path = conv-ttl-blo-hw
 	url = git://ohwr.org/level-conversion/conv-ttl-blo/conv-ttl-blo-hw.git