travis: updated to only build docs for Python 3.6
travis: updated to only build docs for Python 3.6

squaresLab/RepairBox,ChrisTimperley/RepairBox,ChrisTimperley/RepairBox,ChrisTimperley/RepairBox,ChrisTimperley/AutomatedRepairBenchmarks.c,ChrisTimperley/AutomatedRepairBenchmarks.c,ChrisTimperley/AutomatedRepairBenchmarks.c,ChrisTimperley/RepairBox,ChrisTimperley/AutomatedRepairBenchmarks.c,ChrisTimperley/RepairBox,ChrisTimperley/RepairBox,ChrisTimperley/RepairBox,ChrisTimperley/RepairBox,ChrisTimperley/AutomatedRepairBenchmarks.c,ChrisTimperley/AutomatedRepairBenchmarks.c,ChrisTimperley/RepairBox,ChrisTimperley/AutomatedRepairBenchmarks.c,ChrisTimperley/AutomatedRepairBenchmarks.c,ChrisTimperley/AutomatedRepairBenchmarks.c,ChrisTimperley/RepairBox