Update HX711 library

[amotl]

When checking the state of the repository, it looks like it is using an outdated version of the HX711 library [1], which we updated through bogde/HX711#123 the other day.

When bringing this up to speed, all code using the library has to be adjusted to account for the updated constructor, see also [2].

[1] https://github.com/bogde/HX711/tree/e80de1c07e990e07627ec215eec9e1efabf281ee
[2] https://github.com/hiveeyes/HX711/blob/spring-cleaning/README.md#deprecation-warning

[amotl]

We are using PlatformIO everywhere now (GH-58), and pinned the HX711 library to bogde/HX711@0.7.4.