Fix Travis caching

[arm4b]

This should fix cases when travis caches pip installs from https://github.com/StackStorm/st2/blob/master/requirements.txt#L9 when the target master branch code was actually updated (happened in #3721).

Pull Request caches rely first on master branch: https://docs.travis-ci.com/user/caching/#Pull-request-builds-and-caches So we won't cache master branch anymore. It means first build for every PR comes with no cache.

There is also no way to invalidate the cache at the moment once the content is changed in remote git, except of pip upgrade or version change in pip package. See reports in pip:

• pip unexpectedly not installing latest version of git package with branch/commit pinning pypa/pip#2837
• Pip saves vcs-dependency in zip-file with version pypa/pip#2835

Using --editable pip requirements will fix the issue, but it's not supported by dh-virtualenv: spotify/dh-virtualenv#134

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I won't know if the fix actually worked or not before merging this into master and testing #3727 afterwards.