build: update minimum required Meson version

build: update minimum required Meson version

Bump Meson required version to 0.49.2 which is chosen so as
to be provided by both redhat-8 and debian-10.

Update documentation and travis setup script accordingly.

This fixes the following warning:
WARNING: Project targeting '>= 0.47.1' but tried to use feature introduced
         in '0.48.0': console arg in custom_target

'console' argument is used within kernel/linux/kni/meson.build

Signed-off-by: Gabriel Ganne <gabriel.ganne@6wind.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>