Add Maven flag for updating artifacts from snapshot repositories.

[rgrunber]

• With JDT-LS depending on static (eg. latest) repositories, the Tycho build system may end up using stale artifacts cached locally, even when newer ones exist.

[datho7561]

Is it also worth updating in the gulpfile? i.e.

diff --git a/gulpfile.js b/gulpfile.js
index b0ca41a..1bdd4ca 100644
--- a/gulpfile.js
+++ b/gulpfile.js
@@ -282,7 +282,7 @@ function download_server_fn(){

 function build_server_fn(){
    fse.removeSync('./server');
-   cp.execSync(mvnw() + ' -Pserver-distro clean package -Declipse.jdt.ls.skipGradleChecksums', { cwd: server_dir, stdio: [0, 1, 2] });
+   cp.execSync(mvnw() + ' -Pserver-distro clean package -Declipse.jdt.ls.skipGradleChecksums -U ', { cwd: server_dir, stdio: [0, 1, 2] });
    gulp.src(server_dir + '/org.eclipse.jdt.ls.product/distro/*.tar.gz', { encoding: false })
        .pipe(decompress())
        .pipe(gulp.dest('./server'));

(I was using this locally)