Require maven 3.5.2

This prepares builds on Java 9 [1].

The maven 2 tag "prerequisites" is not honored by maven 3
hence use maven-enforcer-plugin to enforce the minimum
maven version.

[1] https://issues.apache.org/jira/browse/MNG-6148

Change-Id: I57f5051a0641b1bd21f9f888f1a17d8f98e879e5
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit.packaging/pom.xml

@@ -119,6 +119,26 @@
 
   <build>
     <plugins>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-enforcer-plugin</artifactId>
+        <version>3.0.0-M1</version>
+        <executions>
+          <execution>
+            <id>enforce-maven</id>
+            <goals>
+              <goal>enforce</goal>
+            </goals>
+            <configuration>
+              <rules>
+                <requireMavenVersion>
+                  <version>3.5.2</version>
+                </requireMavenVersion>
+              </rules>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
       <plugin>
         <groupId>org.eclipse.tycho</groupId>
         <artifactId>tycho-maven-plugin</artifactId>

pom.xml

@@ -70,10 +70,6 @@
     <connection>scm:git:https://git.eclipse.org/r/jgit/jgit</connection>
   </scm>
 
-  <prerequisites>
-    <maven>3.3.1</maven>
-  </prerequisites>
-
   <ciManagement>
     <system>hudson</system>
     <url>https://hudson.eclipse.org/jgit/</url>
@@ -462,6 +458,27 @@
     </pluginManagement>
 
     <plugins>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-enforcer-plugin</artifactId>
+        <version>3.0.0-M1</version>
+        <executions>
+          <execution>
+            <id>enforce-maven</id>
+            <goals>
+              <goal>enforce</goal>
+            </goals>
+            <configuration>
+              <rules>
+                <requireMavenVersion>
+                  <version>3.5.2</version>
+                </requireMavenVersion>
+              </rules>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
+
       <plugin>
         <artifactId>maven-compiler-plugin</artifactId>
       </plugin>