Resolve build script plugins via http vs https

The build script should work against http anyway; use of https here was
an oversight. Changing it now is in response to the following build
failure experienced by a user on his initial attempt to build from
source (with --info output):

  09:02:09.437 [ERROR] [org.gradle.BuildExceptionReporter] Caused
  by: javax.net.ssl.SSLPeerUnverifiedException: peer not authenticated
  ...
  Cause: Could not GET
  https://repo.springsource.org/plugins-snapshot/org/springframework/
  build/gradle/docbook-reference-plugin/0.1.2-SNAPSHOT/maven-metadata.xml

The actual cause is unknown at this time, but worth noting that upon
switching the url to http, the following log message was issued:

  Forcing close on abandoned resource: Http GET Resource:
  http://repo.springsource.org/plugins-snapshot/org/springframework/
  build/gradle/docbook-reference-plugin/0.1.2-SNAPSHOT/maven-metadata.xml

build.gradle

@@ -1,6 +1,6 @@
 buildscript {
     repositories {
-        maven { url 'https://repo.springsource.org/plugins-snapshot' }
+        maven { url 'http://repo.springsource.org/plugins-snapshot' }
     }
     dependencies {
         classpath 'org.springframework.build.gradle:docbook-reference-plugin:0.1.2-SNAPSHOT'