Move away from travis-ci

[probonopd]

Since June 15th, 2021, the building on travis-ci.org is ceased. They recommend travis-ci.com but it says "Builds have been temporarily disabled for public repositories due to a negative credit balance. Please go to the Plan page to replenish your credit balance or alter your Consume paid credits for OSS setting."

It is a pity that OSS projects are now apparently forced to pay for Travis CI.

We need to do something.

[probonopd]

Let's do what https://github.com/arendst/Tasmota/ is doing. Namely, run PlatformIO in GitHub Actions.

Eventually including their incredible gitpod.io trickery which allows one to do customized builds right from the web browser using PlatformIO in no time and with no manual typing of any commands.

[probonopd]

https://github.com/probonopd/WirelessPrinting/runs/3454931801?check_suite_focus=true fails to build:

Run VERSION=$(git rev-parse --short HEAD)
********************************************************************************
If you like PlatformIO, please:
- follow us on Twitter to stay up-to-date on the latest project news > https://twitter.com/PlatformIO_Org
- star it on GitHub > https://github.com/platformio/platformio
- try PlatformIO IDE for embedded development > https://platformio.org/platformio-ide
********************************************************************************

Processing nodemcuv2 (board: nodemcuv2; platform: espressif8266@2.0.0; framework: arduino)
--------------------------------------------------------------------------------
PlatformManager: Installing espressif8266 @ 2.0.0
Error: Detected unknown package 'espressif8266'
Error: Process completed with exit code 1.

Why is this? I thought we were pinning everything...

Do you have an idea @TheAssassin?

[probonopd]

Move complete.