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.
Merge branch 'gh-pages' of https://github.com/spring-projects/gh-pages into gh-pages
Update to 1.2.5

* Drop maven/gradle widget
* Replace with download link
* Add extra link to Ref Docs (PDF)