travis.yml: Limit simultaneous jobs to 3

Even though the host machines that run the Travis CI jobs have
quite a lot of CPUs (e.g. nproc in an aarch64 job reports 32), the
containers on Travis are still limited to 2 vCPUs according to:

 https://docs.travis-ci.com/user/reference/overview/#approx-boot-time

So we do not gain much when compiling with a job number based on
the output of "getconf _NPROCESSORS_ONLN" - quite the contrary, the
aarch64 containers are currently aborting quite often since they
are running out of memory. Thus let's rather use a fixed number
like 3 in the jobs here, so that e.g. two threads can actively run
while a third one might be waiting for I/O operations to complete.
This should hopefully fix the out-of-memory failures in the aarch64
CI jobs.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210217102531.1441557-1-thuth@redhat.com>
[AJB: add comment]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210217121932.19986-6-alex.bennee@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>

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