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.

_includes/widget_templates.html

@@ -62,11 +62,11 @@
 </script>
 
 
+{% if site.custom_pom_template %}
 {% capture maven_pom_template %}
 {% include pom.xml %}
 {% endcapture %}
-
-{%if maven_pom_template contains 'not found in _includes directory' %}
+{% else %}
 {% capture maven_pom_template %}
 <dependencies>
     <dependency>
@@ -99,11 +99,11 @@
 {% endhighlight %}
 </script>
 
+{% if site.custom_gradle_template %}
 {% capture gradle_template %}
 {% include build.gradle %}
 {% endcapture %}
-
-{%if gradle_template contains 'not found in _includes directory' %}
+{% else %}
 {% capture gradle_template %}
 dependencies {
     compile '{@= groupId @}:{@= artifactId @}:{@= version @}'