Update .travis.yml (#1715)

cache pip,
use 'before script' as it done in mmcv,
see https://github.com/open-mmlab/mmdetection/issues/1658

.travis.yml

@@ -7,6 +7,7 @@ python:
   - "3.7"
 
 env: CUDA=10.1.105-1 CUDA_SHORT=10.1 UBUNTU_VERSION=ubuntu1804
+cache: pip
 
 # Ref to CUDA installation in Travis: https://github.com/jeremad/cuda-travis
 before_install:
@@ -27,10 +28,12 @@ install:
   - pip install -r requirements.txt
   - pip install -r tests/requirements.txt
 
-script:
+before_script:
   - flake8 .
   - isort -rc --check-only --diff mmdet/ tools/ tests/
   - yapf -r -d --style .style.yapf mmdet/ tools/ tests/
+
+script:
   - python setup.py check -m -s
   - python setup.py build_ext --inplace
   - coverage run --source mmdet -m py.test -v --xdoctest-modules tests mmdet