* CMake 3.9 is now installed by default on Travis: https://docs.travis-ci.com/user/build-environment-updates/2017-12-12/
* Clang 5.0 (and clang-format, clang-tidy) are now installed on Travis
* Just get clang-format-diff.py
* Move extensive setup to .ci/*
* Fix usage of yaml keys
* Avoid if statements by using build matrix entries
* Note clearly that one block of code will not run
* Remove comments about interleaved macOS

This reverts commit f9e2adf8.
Use the "no fail" approach instead.

Disable analyzer alpha checks yielding a few false positives. Enable all other
stable static analysis checks by default.

Add experimental clang-tidy modernization checks.

Currently, we run only a very small subset of checks (the static analysis checks).

For PRs clang-tidy reports only if the regression is triggered by one of the
modified lines.

There is a cron-job mode which is unused and it could be enabled (through travis)
in future. If enabled now, it reports multitude of issues (reports stop at 10K
lines of log due to travis limit).