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 changes from master
Merge remote-tracking branch 'gh-pages-upstream/gh-pages' into gh-pages
Add custom pom.xml template

Even snapshots of Spring Boot depend on milestones of other projects
at the moment, so we need a custom pom.xml template to bring in the
starter parent (and in turn to declare the repository there).

Half of work for (see master for other) gh-117.