Updates docs to reference 4.0.x

Author: spencergibb

README.adoc

@@ -161,7 +161,7 @@ author credit if we do.  Active contributors might be asked to join the core tea
 given the ability to merge pull requests.
 
 === Code of Conduct
-This project adheres to the Contributor Covenant https://github.com/spring-cloud/spring-cloud-build/blob/master/docs/src/main/asciidoc/code-of-conduct.adoc[code of
+This project adheres to the Contributor Covenant https://github.com/spring-cloud/spring-cloud-build/blob/{spring-cloud-build-branch}/docs/src/main/asciidoc/code-of-conduct.adoc[code of
 conduct]. By participating, you  are expected to uphold this code. Please report
 unacceptable behavior to spring-code-of-conduct@pivotal.io.
 
@@ -172,7 +172,7 @@ added after the original pull request but before a merge.
 * Use the Spring Framework code format conventions. If you use Eclipse
   you can import formatter settings using the
   `eclipse-code-formatter.xml` file from the
-  https://raw.githubusercontent.com/spring-cloud/spring-cloud-build/master/spring-cloud-dependencies-parent/eclipse-code-formatter.xml[Spring
+  https://raw.githubusercontent.com/spring-cloud/spring-cloud-build/{spring-cloud-build-branch}/spring-cloud-dependencies-parent/eclipse-code-formatter.xml[Spring
   Cloud Build] project. If using IntelliJ, you can use the
   https://plugins.jetbrains.com/plugin/6546[Eclipse Code Formatter
   Plugin] to import the same file.

docs/src/main/asciidoc/README.adoc

@@ -9,8 +9,8 @@ include::quickstart.adoc[]
 
 == Building
 
-include::https://raw.githubusercontent.com/spring-cloud/spring-cloud-build/master/docs/src/main/asciidoc/building.adoc[]
+include::https://raw.githubusercontent.com/spring-cloud/spring-cloud-build/4.0.x/docs/src/main/asciidoc/building.adoc[]
 
 == Contributing
 
-include::https://raw.githubusercontent.com/spring-cloud/spring-cloud-build/master/docs/src/main/asciidoc/contributing.adoc[]
+include::https://raw.githubusercontent.com/spring-cloud/spring-cloud-build/4.0.x/docs/src/main/asciidoc/contributing.adoc[]

docs/src/main/asciidoc/spring-cloud-bus.adoc

@@ -3,7 +3,7 @@ include::_attributes.adoc[]
 
 include::intro.adoc[]
 
-include::https://raw.githubusercontent.com/spring-cloud/spring-cloud-build/master/docs/src/main/asciidoc/contributing-docs.adoc[]
+include::https://raw.githubusercontent.com/spring-cloud/spring-cloud-build/4.0.x/docs/src/main/asciidoc/contributing-docs.adoc[]
 
 == Quick Start