use fixed versions of maven-jar-plugin and maven-invoker-plugin (#155)
use fixed versions of maven-jar-plugin and maven-invoker-plugin (#155)

This fixes the Maven warnings which appear on the build:

[WARNING] Some problems were encountered while building the effective model for ch.vorburger.mariaDB4j:mariaDB4j-maven-plugin:maven-plugin:2.3.1-SNAPSHOT
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-jar-plugin is missing. @ ch.vorburger.mariaDB4j:mariaDB4j-maven-plugin:[unknown-version], /home/vorburger/dev/MariaDB4j/git/MariaDB4j/mariaDB4j-maven-plugin/pom.xml, line 181, column 21
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-invoker-plugin is missing. @ ch.vorburger.mariaDB4j:mariaDB4j-maven-plugin:[unknown-version], /home/vorburger/dev/MariaDB4j/git/MariaDB4j/mariaDB4j-maven-plugin/pom.xml, line 259, column 29
[WARNING]
[WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
[WARNING]
[WARNING] For this reason, future Maven versions might no longer support building such malformed projects.
vorburger/MariaDB4j,vorburger/MariaDB4j