Several improvements have been made so that builds run
faster and with fewer canceled jobs:

* Set ccache max size to 1GB. Was 512MB for Linux
(too low for MariaDB) and 5GB on macOS with defaults;

* Don't install libasan in Travis if not necessary.
Sicne ASAN is disabled for the time being, save
time/resources for other steps;

* Decrease number of parallel processes. To prevent
resource exhaustion leading to poor performance. According
to Travis docs, a max of 4 concurrent processses should be
run per job:
https://docs.travis-ci.com/user/common-build-problems/#My-build-script-is-killed-without-any-error

* Reconsider tests exec order and split huge main and rocksdb
test suites into their own job, decreasing the chance of going
over the Travis job execution limit and getting killed;

* Increase Travis testcase-timeout to 4 minutes. Occasionally
on Ubuntu target and frequently on macOS, many tests in main,
rpl, binlog suites take longer than 2 minutes, resulting in
many jobs failing, when in reality the failing tests didn't
get a chance to complete. From my testing, along with the other
speedups, i.e. increasing ccache size, a timeout of 4 minutes
should be Ok.  Revert to 3 minutes of necessary.

* Build with GCC and Clang version 5,6 only.

* Rename GCC_VERSION to CC_VERSION for clarity. We are using
two compilers after all, GCC and Clang.

* Stop using somewhat obsolete Clang4 in Travis. Also, was the
reason for the failing test suites in MDEV-15430.