travis: unlink installed pkg-config
travis: unlink installed pkg-config

pkg-config 0.29 was merged in Homebrew 4 days ago.
This causes test failure on Travis CI's OSX instances.

When pkg-config 0.28 is installed on OS X with Homebrew
and then installing pkg-config 0.29:

NG:

``` bash
$ brew install pkg-config
```

OK:

```bash
$ brew unlink pkg-config
$ brew install pkg-config
```

or users can install pkg-config with `brew upgrade pkg-config`.

Above commands are tested on my MacBook '12 El Capitan.

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