switch parent pom reference everywhere to 28 where it was 27 before
this can be safely done because the only difference between 27 and 28 is an updated maven-source-plugin (fix heap space error)

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1756968 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/pom.xml b/pom.xml
index e5c7068..ec97e97 100644
--- a/pom.xml
+++ b/pom.xml
@@ -22,7 +22,7 @@
     <parent>
         <groupId>org.apache.sling</groupId>
         <artifactId>sling</artifactId>
-        <version>27</version>
+        <version>28</version>
         <relativePath />
     </parent>