test:cbor: supress warnings about nan/infty
Travis: enable apt caching

This should speed up the installation process a bit.
tests: run cppcheck for PR in Travis

Failures are ignored for now.
Merge pull request #1432 from LudwigOrtmann/cppcheck

tests: enable cppcheck for PR in travis
Merge pull request #1689 from LudwigOrtmann/issue-1636-p1

Travis: enable apt caching
Travis: merge install commands
Merge pull request #1691 from LudwigOrtmann/issue-1636-p2

Travis: merge install commands
Merge pull request #1688 from N8Fear/naninarith

lint: supress warnings about nan/infty in arithmetics in the CBOR unit tests