travis: clean up

[proppy]

• use ${VAR} everywhere
• use wget everywhere
• use unzip -q everywhere
• use the same library install procedure that @ed7coyne introduced for SoftwareSerial

[ed7coyne]

LGTM assuming travis is happy.

We need to do a real cleanup though at some point. The whole file seems so ugly, I am not sure if it is possible to have a clean travis.yml but maybe we can make it happen eventually.

[proppy]

@ed7coyne any reason you added -q everywhere, I kinda like having a verbose output for debugging and travis ui collapse the log per commands by default