Update Software Serial

[peteruithoven]

Currently we use a SoftwareSerial fork with some tweaks to allow half duplex communication, but this was last changed in 2012. I would like to use the latest SoftwareSerial, so I've done a pull request for half duplex support:
arduino/Arduino#4377

[peteruithoven]

After doing some research I understand that the circuit we use (see below) makes that software half duplex feature redundant. The regular SoftwareSerial didn't work because we switched the rx and tx pins...
https://github.com/knopjesmuseum/resilient-network/blob/master/pcb/half-duplex-circuit.png

We can now use the regular SoftwareSerial (without having to enable the inverse logic).