Cross-library include fails due to library renaming

See #64; some of the library header files try to get the LMIC configuration by directly looking for `../../arduino-lmic/src/lmic/lmic_config_preconditions.h`.  But `arduino-lmic` isn't the name when installed by the IDE.

Instead, we need to use the new header file (from 2.3.1 of the LMIC), <arduino_lmic_user_configuration.h>.
