Use isort to sort imports and setup travis (#1085)

.isort.cfg

.travis.yml

mmdet/apis/train.py

mmdet/core/anchor/anchor_target.py

mmdet/core/anchor/guided_anchor_target.py

mmdet/core/bbox/assigners/approx_max_iou_assigner.py

mmdet/core/bbox/assigners/max_iou_assigner.py

mmdet/core/bbox/bbox_target.py

mmdet/core/bbox/samplers/combined_sampler.py

mmdet/core/bbox/samplers/ohem_sampler.py

mmdet/core/evaluation/eval_hooks.py

mmdet/core/fp16/hooks.py

mmdet/core/mask/mask_target.py

mmdet/core/post_processing/merge_augs.py

mmdet/core/utils/dist_utils.py

mmdet/datasets/custom.py

mmdet/datasets/loader/build_loader.py

mmdet/datasets/loader/sampler.py

mmdet/models/anchor_heads/anchor_head.py

mmdet/models/anchor_heads/fcos_head.py