Increase accuracy of mg/dl to mmol/l ratio

[cpitchford]

Using a ratio of 18 mg/ml to 1 mmol/l causes NightScout to report different values compared Dexcom app/receivers.

A more accurate value is based on 0.0555 mmol/l to 1 mg/dl. The inverse is 18.018018018 mg/dl to 1 mmol/l

This puts the value closer to the Dexcom app/receiver. Affected values:

mg/dl	18	18.018018018
109	6.1	6.0
118	6.6	6.5
127	7.1	7.0
136	7.6	7.5
145	8.1	8.0
154	8.6	8.5
163	9.1	9.0
172	9.6	9.5
181	10.1	10.0
190	10.6	10.5
199	11.1	11.0
208	11.6	11.5
217	12.1	12.0
226	12.6	12.5
235	13.1	13.0
244	13.6	13.5
253	14.1	14.0
262	14.6	14.5
271	15.1	15.0
280	15.6	15.5
289	16.1	16.0
298	16.6	16.5
307	17.1	17.0
309	17.2	17.1
316	17.6	17.5
318	17.7	17.6
325	18.1	18.0
327	18.2	18.1
334	18.6	18.5
336	18.7	18.6
343	19.1	19.0
345	19.2	19.1
352	19.6	19.5
354	19.7	19.6
361	20.1	20.0
363	20.2	20.1
370	20.6	20.5
372	20.7	20.6
379	21.1	21.0
381	21.2	21.1
388	21.6	21.5
390	21.7	21.6
397	22.1	22.0
399	22.2	22.1