[travis] Test against JDK 7 and JDK 8
[travis] Test against JDK 7 and JDK 8

Bug #221 was due to a different behaviour of JDK8 and JDK7. To prevent
this in the future, travis should test with both.

ivanovlev/Gadgetbridge,roidelapluie/Gadgetbridge,rosenpin/Gadgetbridge,Freeyourgadget/Gadgetbridge,ivanovlev/Gadgetbridge,Freeyourgadget/Gadgetbridge,rosenpin/Gadgetbridge,roidelapluie/Gadgetbridge,Freeyourgadget/Gadgetbridge,rosenpin/Gadgetbridge,roidelapluie/Gadgetbridge,Freeyourgadget/Gadgetbridge,ivanovlev/Gadgetbridge