build,.travis.yml: drop xcode 8 build
build,.travis.yml: drop xcode 8 build

Builds for Xcode 8 take up about 30+ minutes.
The build also reports:
```
Warning: You are using macOS 10.11.
We (and Apple) do not provide support for this old version.
```

Maybe it's a good time to remove.

Signed-off-by: Alexandru Ardelean <685557f6b7c225d86b536740313ed51b24843a58@analog.com>

analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio