XD-2270 Upgrading to Spring Boot 1.2.0.RC1

- adding showBanner(false) due to Boot issue #1918
XD-2340 Build: Ensure that branch-specific documentation is pulled and generated

* Update `build.gradle`
* Add ability to specify a custom wiki branch

````
    $ ./gradlew pullDocs -PwikiBranch=origin/mycustombranch
````

* By default the doc branch is the same as the XD branch

Jira: https://jira.spring.io/browse/XD-2340

* Address Code review comments
XD-2352 : Property replacement does not happen in XML class attribute

Temporary workaround
XD-2354 Adding property to avoid template check

- The GroovyAutoconfiguration now look for a classpath:template/ directory, need to disable this check