Bump swipl to 7.5.15, set default lang to UTF-8.

library/swipl

@@ -1,6 +1,6 @@
 Maintainers: Dave Curylo <[email protected]> (@ninjarobot)
 GitRepo: https://github.com/SWI-Prolog/docker-swipl.git
 
-Tags: latest, 7.5.14
-GitCommit: b76acfff26b916351287a562d98b29c199c188ab
-Directory: 7.5.14/stretch
+Tags: latest, 7.5.15
+GitCommit: 31e2158e7ba2bb2fa9ebff7c0717c476244506bb
+Directory: 7.5.15/stretch