build,meta: switch to gcc-4.9 on travis

The version of `clang` provided in the Travis linux image uses
libstdc++4.8 whice is below our minimal supported version.

Switching to `make test -j1` is to avoid races during the test cycle
causes by the main target being "unstable", that is it always builds
some files, and relinks the binary, which is used by the test procedure.

PR-URL: https://github.com/nodejs/node/pull/23778


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>