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

mmdet/models/anchor_heads/ga_retina_head.py

mmdet/models/anchor_heads/ga_rpn_head.py

mmdet/models/anchor_heads/guided_anchor_head.py

mmdet/models/anchor_heads/retina_head.py

mmdet/models/anchor_heads/rpn_head.py

mmdet/models/anchor_heads/ssd_head.py

mmdet/models/backbones/hrnet.py

mmdet/models/backbones/resnet.py

mmdet/models/backbones/resnext.py

mmdet/models/backbones/ssd_vgg.py

mmdet/models/bbox_heads/bbox_head.py

mmdet/models/bbox_heads/convfc_bbox_head.py

mmdet/models/builder.py

mmdet/models/detectors/base.py

mmdet/models/detectors/cascade_rcnn.py

mmdet/models/detectors/fast_rcnn.py

mmdet/models/detectors/faster_rcnn.py

mmdet/models/detectors/fcos.py

mmdet/models/detectors/grid_rcnn.py

mmdet/models/detectors/htc.py