The build variants enable group of checks offered by clang
UndefinedBehaviorSanitizer detector.
Add sanitizers suppression file in .ci/blacklist.supp based on
copy of the file from Boost.Beast.
Update Travis CI:
- Add .ci/build-and-test.sh script as handy proxy for b2 command.
- Add build jobs for each of the three new UBSan variants.
- Display COMPILER and VARIANT first as these two are most important
details while inspecting the build matrix
(TOOLSET can be derived from COMPILER).
