Fixed Travis CI python module path problem

Since [Ubuntu doesn't seem to install modules on python 3 default sys.path](https://bugs.launchpad.net/ubuntu/+source/automake/+bug/1250877), python fails to find the installed `cangjie` module (pycangjie).
Travis, hence, fails to build and test the package.

By manually adding the Ubuntu python module path to environment
parameter `$PYTHONPATH`, this problem is gone.