Tag (Alias).valueOf(String) with JsonCreator

[dansanduleac]

Otherwise the method won't be used by newer jackson-databind e.g. 2.9.6

Fixes #13

FLUPs:

• ability to run all tests against multiple versions of jackson Run serialization tests against multiple jackson versions #20

[markelliot]

this shouldn't be required

[dansanduleac]

@melliot tested with jackson 2.9.6 and it doesn't work. Not sure if it's a bug in that version of jackson or a deliberate change (couldn't find it in the changelog) but shouldn't we be extra clear with our intention of what is a JsonCreator?

[dansanduleac]

This is broken by FasterXML/jackson-databind@eb996aa as @robert3005 found.
I'll add a test that fails and the fix on top.

[dansanduleac]

Acually, looking at https://github.com/palantir/conjure/blob/develop/wire.md, this maintains behaviour that used to work, but that we never committed to supporting.
Given that only one product internally seems to be relying on this, and the fix on their side would be easy, we can probably ask them to stop relying on unspecced behaviour.