Update supported OTP versions (18-26) in README #97

Job	Run time
Test front-end	48s
Test different json lib (jiffy \| OTP 19)	3m 17s
OTP 26	1m 20s
Test different json lib (jsx \| OTP 19)	3m 17s
Test with Elixir (Elixir 1.5.x \| OTP 20)	1m 1s
OTP 25	1m 21s
Test different json lib (thoas \| OTP 19)	23s
Test with Elixir (Elixir 1.6.x \| OTP 20)	1m 11s
OTP 24	1m 22s
Test different json lib (jiffy \| OTP 21)	3m 31s
Test with Elixir (Elixir 1.7.x \| OTP 20)	1m 5s
OTP 23	1m 30s
Test different json lib (jsx \| OTP 21)	3m 28s
Test with Elixir (Elixir 1.8.x \| OTP 20)	1m 16s
OTP 22	1m 27s
Test with Elixir (Elixir 1.9.x \| OTP 20)	1m 7s
Test different json lib (thoas \| OTP 21)	3m 20s
OTP 21	3m 30s
Test with Elixir (Elixir 1.10.x \| OTP 21)	1m 5s
OTP 20	1m 22s
Test with Elixir (Elixir 1.11.x \| OTP 22)	1m 11s
OTP 19	1m 15s
Test with Elixir (Elixir 1.12.x \| OTP 23)	1m 16s
OTP 19	1m 9s
Test with Elixir (Elixir 1.13.x \| OTP 24)	1m 32s
OTP 18	1m 25s
Test with Elixir (Elixir 1.14.x \| OTP 25)	1m 12s
Test with Elixir (Elixir 1.15.x \| OTP 26)	1m 13s
	46m 54s

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