travis: use both cores for make/make check

The instance offers 2 cores, so use them to speed things up.

v2: Set MAKEFLAGS instead [Eric]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

.travis.yml

@@ -37,6 +37,7 @@ env:
     - LIBXCB_VERSION=libxcb-1.11
     - LIBXSHMFENCE_VERSION=libxshmfence-1.2
     - PKG_CONFIG_PATH=$HOME/prefix/lib/pkgconfig
+    - MAKEFLAGS=-j2
   matrix:
     - BUILD=make
     - BUILD=scons