Add CC2538 travis compile tests
Merge pull request #320 from g-oikonomou/travis-cc2538

Add CC2538 travis compile tests
Merge pull request #306 from adamdunkels/push/travis-mspgcc-4-7-0

Update travis to use mspgcc 4.7.0
6lowpan fragmentation bugfix: the 6lowpan code had an unfortunate
interaction with the behavior of the rdc layer. If the first packet of a
fragment transmission was lost, the remaining packets would get dropped
on reception. Moreover, the reception code contained a bug that
sometimes would cause fragments to be misidentified as fragments. Taken
together, these problems would result in a pathelogical network
breakdown if too many fragmented packets would occur simultaneously.
Bugfix: must set the SICSLOWPAM_MAX_MAC_TRANSMISSIONS attribute after clearing the buffer, otherwise it will not be used for outoing packets.
Added regression tests that test IPv6 fragmentation