[Mod] move requirements-travis.txt to /travis/folder

[Add] added format check

.travis.yml

@@ -1,12 +1,12 @@
 language: python
 python:
-  - "3.7"
+  - "3.7-dev"
 install:
   - pip install -r requirements.txt
-  - pip install -r requirements-travis.txt
+  - pip install -r travis/requirements.txt
   - pip install -r requirements-dev.txt
 # command to run tests
 script:
-  - yapf --style .style.yapf
+  - python travis/format_check.py
   - pylint --rcfile=.pylintrc vnpy
   - python tests/test_all.py