Add support for jackson 2.12

[ghillairet]

Fix for #6

[juergen-albert]

I gave it a try and it seems to do the job. @ghillairet thanks heaps for fixing this!

[PhilippeBrossel]

I tried it out, and your fix solves the issue for me.
@ghillairet : Many thanks for this great tool !
Btw, I had to add a /META-INF/MANIFEST.MF with an adequate Export-Package to be able to access the packages org.emfjson.jackson.* from my plugin-projects
(Was just to circumvent the warning This plug-in does not have an OSGI bundle manifest.)
Is there a better way to do that or can you give me a hint of what I'm doing wrong ?

[debovema]

Can someone merge this ?

[martin-fleck-at]

Thank you very much for the fix, @ghillairet . Btw, we are in the process of setting up the build infrastructure for this so it might not be immediately consumable right now.