slapos-sr-testing: Use slapos.core 1.4.15 for old slapos proxy

Since slapos.core 1.4.16+ slapos proxy is not able to consume or publish
non-strings, which makes tests not working.

software/slapos-sr-testing/software.cfg

@@ -134,6 +134,9 @@ output = ${buildout:directory}/template.cfg
 mode = 640
 
 [versions]
+# Use slapos.core <= 1.4.16 for correctly working slapos proxy
+slapos.core = 1.4.15
+
 # clear the version of tested eggs, to make sure we installed the developped ones
 erp5.util =