export path not effective on travis xenial image

https://github.com/oursky/git-travis-change/blob/master/install#L7

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From [Travis Doc](https://docs.travis-ci.com/user/installing-dependencies/#installing-projects-from-source): 
```
Note that when you’re updating the $PATH environment variable, that part can’t be moved into a shell script, as it will only update the variable for the sub-process that’s running the script.
```