Drop JDK 6 from Travis configuration.
Drop JDK 6 from Travis configuration.

WorstCase00/jgrapht,cthiebaud/jgrapht,wselwood/jgrapht,Infeligo/jgrapht,WorstCase00/jgrapht,gjroelofs/jgrapht,WorstCase00/jgrapht,arcanefoam/jgrapht,feilong0309/jgrapht,alexeykudinkin/jgrapht,alexeykudinkin/jgrapht,cthiebaud/jgrapht,AidanDelaney/jgrapht,mt0803/jgrapht,Infeligo/jgrapht,WorstCase00/jgrapht,mt0803/jgrapht,feilong0309/jgrapht,AidanDelaney/jgrapht,gjroelofs/jgrapht,kashak79/jgrapht,wselwood/jgrapht,kashak79/jgrapht,arcanefoam/jgrapht