Switch to openmrs core version 2.6.15

Author: dkayiwa

distro/pom.xml

@@ -25,7 +25,7 @@
          we do so here so that we can utilise Maven to track updates, etc. -->
     <fhir2.version>2.3.0-SNAPSHOT</fhir2.version>
     <authentication.version>1.0.0</authentication.version>
-    <openmrs.version>2.6.14-SNAPSHOT</openmrs.version>
+    <openmrs.version>2.6.15</openmrs.version>
     <initializer.version>2.8.0</initializer.version>
     <webservices.rest.version>2.48.0-SNAPSHOT</webservices.rest.version>
     <addresshierarchy.version>2.19.0</addresshierarchy.version>