Make versionsLT support versions with qualifier (#307)

Extracted functions to jenkins-support; included in both jenkins.sh and
install-plugin.sh.

Added a separate test file to test versionLT function
diff --git a/Dockerfile b/Dockerfile
index 98b6bf5..4a0bdcd 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -58,6 +58,7 @@
 
 USER ${user}
 
+COPY jenkins-support /usr/local/bin/jenkins-support
 COPY jenkins.sh /usr/local/bin/jenkins.sh
 ENTRYPOINT ["/bin/tini", "--", "/usr/local/bin/jenkins.sh"]