Artifacts should be downloaded using https

See http://blog.ontoillogical.com/blog/2014/07/28/how-to-take-over-any-java-developer/

readme.txt

@@ -44,7 +44,7 @@ MAVEN REPOSITORY DOWNLOADS
 
 Release jars for the project are available from the central maven repository
 
-http://repo1.maven.org/maven2/org/springframework/security/
+https://repo1.maven.org/maven2/org/springframework/security/
 
 Note that milestone releases and snapshots are not uploaded to the central
 repository, but can be obtained from the Spring milestone repository, using the
@@ -78,4 +78,4 @@ CONTRIBUTING
 
 Contributions are welcome. Please refer to the Contributor Guidelines for details
 
-  https://github.com/SpringSource/spring-security/wiki/Contributor-Guidelines
+  https://github.com/SpringSource/spring-security/wiki/Contributor-Guidelines