Convert the continuous integration process to use GitHub's internal
CI cloud. Allows us to run up to 20 jobs in parallel, speeding up
CI immensely. Keep a short Travis-CI run, too, just for sanity and backup.
Uses new keys and secret when publishing a release to
esp8266.github.io.
* remove lwip-v1.4 specific code
* ditto
* ditto
* fix ip4_addr definition
* CI: change debug builds to use IPv6, remove regular IPv6 builds
* ditto
* split pio CI in four (because they last twice the time of the other builds)
* remove option from pio
* remove lwIP-1.4 from doc
* restore pio CI splitting
* fix CI debug6 script
* ditto
