v4.0.0 release: assign channels without replacement (& other fixes)

This is a major release, and (since it changes some of the contents of the LMIC structure, which are public) potentially breaking.

Potentially breaking changes include:

• Remove XCHANNEL support from US region #404
• Assign channels randomly without replacement #515, #619, #730.
• Don't allow LMIC_setupChannel() to change default channels #714
• Adopt semantic versions completely #726.

Enhancements include:

• Add LMIC_queryNumDefaultChannels() #700.
• Don't compile board config objects when we know for sure they'll not be used; compilers can't always tell #736.

Important bug-fixes include:

• Don't accept out-of-range DRs from MAC downlink messages #723
• Implement JoinAccept CFList processing for US/AU #739.
• Correct JoinAccept CFList processing for AS923 #740.

Fixes include:

• Fix some broken documentation references #644, #646, #673.
• Re-added CI testing, since Travis CI no longer works for us #647; fixed AVR compliance CI compile #679.
• Don't use defined() in macro definitions #606
• Fix a warning on AVR32 #709.
• Fix Helium link in examples #715, #718.