Install plugins even when they already exist in a different version (#323)

diff --git a/tests/install-plugins/update/Dockerfile b/tests/install-plugins/update/Dockerfile
new file mode 100644
index 0000000..c088223
--- /dev/null
+++ b/tests/install-plugins/update/Dockerfile
@@ -0,0 +1,3 @@
+FROM bats-jenkins-install-plugins
+
+RUN /usr/local/bin/install-plugins.sh maven-plugin:2.13 ant:1.3