Add Zygote again to resolve CI issues

Author: Alex Robson

Manifest.toml

@@ -27,6 +27,12 @@ git-tree-sha1 = "ad613c934ec3a3aa0ff19b91f15a16d56ed404b5"
 uuid = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
 version = "1.0.2"
 
+[[ChainRulesTestUtils]]
+deps = ["ChainRulesCore", "Compat", "FiniteDifferences", "LinearAlgebra", "Random", "Test"]
+git-tree-sha1 = "1886ca03bd02f1ff449c3a08bc8f41584fb5bb25"
+uuid = "cdddcdb0-9152-4a09-a978-84456f9df70a"
+version = "1.0.0"
+
 [[CommonSubexpressions]]
 deps = ["MacroTools", "Test"]
 git-tree-sha1 = "7b8a93dba8af7e3b42fecabf646260105ac373f7"
@@ -81,6 +87,12 @@ git-tree-sha1 = "8c8eac2af06ce35973c3eadb4ab3243076a408e7"
 uuid = "1a297f60-69ca-5386-bcde-b61e274b549b"
 version = "0.12.1"
 
+[[FiniteDifferences]]
+deps = ["ChainRulesCore", "LinearAlgebra", "Printf", "Random", "Richardson", "StaticArrays"]
+git-tree-sha1 = "10d58852f13fbe6fb85cc20846c2753079f5fffa"
+uuid = "26cc04aa-876d-5657-8c51-4c34ba976000"
+version = "0.12.17"
+
 [[ForwardDiff]]
 deps = ["CommonSubexpressions", "DiffResults", "DiffRules", "LinearAlgebra", "NaNMath", "Printf", "Random", "SpecialFunctions", "StaticArrays"]
 git-tree-sha1 = "e2af66012e08966366a43251e1fd421522908be6"
@@ -181,6 +193,12 @@ git-tree-sha1 = "4036a3bd08ac7e968e27c203d45f5fff15020621"
 uuid = "ae029012-a4dd-5104-9daa-d747884805df"
 version = "1.1.3"
 
+[[Richardson]]
+deps = ["LinearAlgebra"]
+git-tree-sha1 = "e03ca566bec93f8a3aeb059c8ef102f268a38949"
+uuid = "708f8203-808e-40c0-ba2d-98a6953ed40d"
+version = "1.4.0"
+
 [[SHA]]
 uuid = "ea8e919c-243c-51af-8825-aaa63cd721ce"