Update to Tycho 0.20.0, include versions plugin.

Author: ultraq

pom.xml

@@ -80,7 +80,7 @@
 	</pluginRepositories>
 
 	<properties>
-		<tycho.version>0.18.0</tycho.version>
+		<tycho.version>0.20.0</tycho.version>
 		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
 		<maven.build.timestamp.format>yyyyMMddHHmm</maven.build.timestamp.format>
 		<thymeleaf.plugin-name>thymeleaf-extras-eclipse-plugin</thymeleaf.plugin-name>
@@ -121,6 +121,11 @@
 					</environments>
 				</configuration>
 			</plugin>
+			<plugin>
+				<groupId>org.eclipse.tycho</groupId>
+				<artifactId>tycho-versions-plugin</artifactId>
+				<version>${tycho.version}</version>
+			</plugin>
 			<plugin>
 				<groupId>org.eclipse.tycho</groupId>
 				<artifactId>tycho-compiler-plugin</artifactId>