travis: separate lint build

[tooomm]

Short roundup of the initial problem

Old Xenial build was only used to run our tests.
Lint was combined with that same build and reporting back took ~5min.

What will change with this Pull Request?

• Have separate lint build, reports back in 15s
  I created a separate lint script for that purpose. The --format argument can still be passed to the compile script which will execute the new script then.
• Move --test to the Bionic build
• Remove the old (and slower) Xenial build, we only need Bionic

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Thanks for your help @ebbit1q!