Remove 'not found in _includes' hacks

Remove the use if "unless contains 'not found in _includes directory'"
hacks that were previously used to detect when _includes were missing.

This change is required as the hack no longer works with the latest
Jekyll release (and therefore also with GitHub).

Additional 'custom_pom_template' and 'custom_gradle_template' flags
must now be set in _config.yml if you want to include a custom pom.xml
or build.gradle file.

_config.yml

@@ -25,6 +25,10 @@ github_repo_url: http://github.com/spring-projects/spring-framework
 # Project forum URL
 forum: http://forum.spring.io/forum/spring-projects/container
 
+# If you want to include a custom pom.xml or gradle template set these value to true and add _include files
+custom_pom_template: false
+custom_gradle_template: false
+
 
 ### The following properties are constant for most projects