[Misc]: Speed up travis-ci by wheel

[mike820324]

We should use wheel in travis-ci to speed up the process.

Reference:

• http://atchai.com/blog/faster-ci/
• https://tzangms.com/how-to-speed-up-python-unit-test-on-travis-ci/ (chinese)

[chhsiao90]

First, we can simply add pip cache https://docs.travis-ci.com/user/caching/#pip-cache To prevent the downloading from internet.