travis: add "scons swr" to the build matrix

travis: add "scons swr" to the build matrix

Requires GCC 5.0 (due to the C++14 requirement) and LLVM 3.9.

v2: Enable the target, add libedit-dev, rework check target.
v3: Comment the current check target, add -j4 SCONSFLAGS, quote OVERRIDE
variables.
v4: Keep check target as-is (Andres)

Cc: Tim Rowley <timothy.o.rowley@intel.com>
Cc: George Kyriazis <george.kyriazis@intel.com>
Reviewed-by: George Kyriazis <george.kyriazis@intel.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Andres Gomez <agomez@igalia.com>