change Antlr4Plugin to allow the user to specify alternate antlr versions

Cédric,

Many thanks for making your project open-source.  It is clean and simple.

However I need a clean way to override the ANTLR4 version so that I and/or users can use development and/or future releases without having to change the plugin.

I have done this as cleanly as I could.

I use a project.afterEvaluate in which I check the antlr4 configuration to see if the user has added a dependency... and if they have not, I add the current default 'org.antlr:antlr4:4.2.2' version.

I have also added two additional tests to ensure the default dependency is added if the user has not added their own dependency/version.

It would be very useful to the whole community if you could accept this pull request.

Let me know if you have any questions or changes.

Regards, Stephen Gaito (lead/only developer of FandianPF)