GHC 8.6.1 support (#95)

* GHC 8.6.1 support

 - Bounds changes for base, containers and stm
 - stack lts 12.* tests
 - works with stack 8.6.1 nightly
    stack build --resolver nightly --haddock --test --bench --no-run-benchmarks
 - works with cabal new-build

* GHCVER-8.6.1

protolude.cabal

@@ -1,5 +1,5 @@
 name:                protolude
-version:             0.2.2
+version:             0.2.4
 synopsis:            A small prelude.
 description:         A sensible set of defaults for writing custom Preludes.
 homepage:            https://github.com/sdiehl/protolude
@@ -25,7 +25,8 @@ tested-with:
   GHC == 7.10.3,
   GHC == 8.0.1,
   GHC == 8.2.1,
-  GHC == 8.4.1
+  GHC == 8.4.1,
+  GHC == 8.6.1
 extra-source-files: README.md CHANGES.md
 
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