Streamline versions-mojo-version definition

Make it a property so updates are done together for different artifacts
coming from it.

(cherry picked from commit 4ac9f1a)

tycho-extras/tycho-version-bump-plugin/pom.xml

@@ -27,6 +27,9 @@
 		<maven>${minimal-maven-version}</maven>
 	</prerequisites>
 	<name>Tycho Version Bump Plugin</name>
+	<properties>
+		<versions-mojo-version>2.18.0</versions-mojo-version>
+	</properties>
 
 	<dependencies>
 		<dependency>
@@ -44,12 +47,12 @@
 		<dependency>
 			<groupId>org.codehaus.mojo.versions</groupId>
 			<artifactId>versions-model</artifactId>
-			<version>2.18.0</version>
+			<version>${versions-mojo-version}</version>
 		</dependency>
 		<dependency>
 			<groupId>org.codehaus.mojo.versions</groupId>
 			<artifactId>versions-common</artifactId>
-			<version>2.18.0</version>
+			<version>${versions-mojo-version}</version>
 		</dependency>
 	</dependencies>