Adopted .travis.xml from image-builder-rpi

[docbobo]

Not sure if this is the right thing to do, but this PR picks up two things from .travis.yml as present in image-builder-rpi:

• skip-cleanup: true- this is definitely important according to Travis CI doc.
• api_key - not sure if the same value as in image-builder-rpiwill work here.

[docbobo]

@DieterReuter I hope this fixes the deploy issues. Do you feel this looks right?

[StefanScherer]

@docbobo I simplified the .travis.yml and reorganized deployment in background.

[StefanScherer]

LGTM