CompatHelper: bump compat for AbstractPPL to 0.6 for package test, (k…

…eep existing compat) (#441)

This pull request changes the compat entry for the `AbstractPPL` package from `0.5.1` to `0.5.1, 0.6` for package test.
This keeps the compat entries for earlier versions.



Note: I have not tested your package with this new compat entry.
It is your responsibility to make sure that your package tests pass before you merge this pull request.

test/Project.toml

@@ -21,7 +21,7 @@ Zygote = "e88e6eb3-aa80-5325-afca-941959d7151f"
 
 [compat]
 AbstractMCMC = "2.1, 3.0, 4"
-AbstractPPL = "0.5.1"
+AbstractPPL = "0.5.1, 0.6"
 Bijectors = "0.9.5, 0.10"
 Distributions = "0.25"
 DistributionsAD = "0.6.3"