Bump System.Collections.Immutable and Microsoft.NETFramework.ReferenceAssemblies #555

Job	Run time
Build and Test (debug, net35, windows-latest)	7m 2s
Build and Test (debug, net40, windows-latest)	6m 41s
Build and Test (debug, All, windows-latest)	12m 9s
Build and Test (debug, All, ubuntu-latest)	5m 17s
Build and Test (debug, All, macos-latest)	8m 18s
Build and Test (release, net35, windows-latest)	6m 13s
Build and Test (release, net40, windows-latest)	6m 24s
Build and Test (release, All, windows-latest)	11m 23s
Build and Test (release, All, ubuntu-latest)	5m 20s
Build and Test (release, All, macos-latest)	6m 16s
Event File	2s
Validate generated files	4m 0s
	1h 19m 5s

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