build,travis: some tunings

- use 'git diff --name-only'
- unpack the sdk only once as if there is no errors we don't need clean sdk
  fresh sdk mean new git checkout of the feeds each time
- only include base, packages and luci,
  as we should not have dependencies on other feeds
- use github for feeds
- continue to test on error
- add a bit of color
- use bash

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>

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