Remove previousVersion from 'jdiff' Gradle task

gradle.properties

-previousVersion=3.2.0.RC2
 version=3.2.0.BUILD-SNAPSHOT

gradle/jdiff.gradle

@@ -2,12 +2,12 @@
  * Generate a JDiff report between the current version and an older version.
  *
  * Usage:
- *  gradle jdiff [-D OLD_VERSION=3.1.3.RELEASE] -D OLD_VERSION_ROOT=/path/to/3.1.3.RELEASE
+ *  gradle jdiff -D OLD_VERSION=3.1.3.RELEASE -D OLD_VERSION_ROOT=/path/to/3.1.3.RELEASE
  *
  * View generated report at:
  *  build/reports/jdiff/changes.html
  *
- * @param OLD_VERSION optional, defaulting to value of `previousVersion` in gradle.properties
+ * @param OLD_VERSION required
  * @param OLD_VERSION_ROOT required, typically pointing to a separate git clone dir
  */
 task jdiff {
@@ -21,17 +21,18 @@ task jdiff {
 			classname: "jdiff.JDiffAntTask",
 			classpath: "${jdiffHome}/antjdiff.jar")
 
-	def previousVersion = rootProject.previousVersion
-
 	def currentVersion = rootProject.version
 	def currentVersionRoot = rootProject.rootDir
 
-	def oldVersion = System.getProperty("OLD_VERSION") ?: previousVersion
+	def oldVersion = System.getProperty("OLD_VERSION")
 	def oldVersionRoot = System.getProperty("OLD_VERSION_ROOT")
 
 	def outputDir = "${rootProject.buildDir}/reports/jdiff/${oldVersion}_to_${currentVersion}"
 
 	doLast {
+		if (oldVersion == null)
+			throw new IllegalArgumentException(
+				"Set OLD_VERSION property to indicate older of the two versions being compared")
 		if (oldVersionRoot == null)
 			throw new IllegalArgumentException(
 				"Set OLD_VERSION_ROOT property to indicate the root directory for ${oldVersion}")