Fixes to support OTP 24..26 and Elixir 1.13..1.15 #95

Job	Run time
Test different json lib (jiffy \| OTP 19)	3m 21s
Test front-end	39s
Test different json lib (jsx \| OTP 19)	3m 11s
Test with Elixir (Elixir 1.5.x \| OTP 20)	58s
OTP 26	1m 18s
Test different json lib (thoas \| OTP 19)	22s
Test with Elixir (Elixir 1.6.x \| OTP 20)	1m 8s
Test different json lib (jiffy \| OTP 21)	3m 36s
OTP 25	1m 26s
Test with Elixir (Elixir 1.7.x \| OTP 20)	1m 5s
OTP 24	1m 23s
Test different json lib (jsx \| OTP 21)	3m 26s
Test with Elixir (Elixir 1.8.x \| OTP 20)	1m 4s
OTP 23	1m 25s
Test different json lib (thoas \| OTP 21)	3m 22s
Test with Elixir (Elixir 1.9.x \| OTP 20)	1m 11s
OTP 22	1m 21s
Test with Elixir (Elixir 1.10.x \| OTP 21)	1m 3s
OTP 21	3m 29s
Test with Elixir (Elixir 1.11.x \| OTP 22)	1m 7s
OTP 20	1m 20s
Test with Elixir (Elixir 1.12.x \| OTP 23)	1m 12s
OTP 19	1m 19s
Test with Elixir (Elixir 1.13.x \| OTP 24)	1m 9s
OTP 19	1m 11s
Test with Elixir (Elixir 1.14.x \| OTP 25)	1m 11s
OTP 18	1m 18s
Test with Elixir (Elixir 1.15.x \| OTP 26)	1m 6s
	45m 41s

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