.travis.yml: make -j3

The move from Travis VMs to Containers came with a upgrade from 1.5
cores to 2. The received wisdom is -j N+1 means a core can be doing work
while other threads wait for IO to complete. This is hard to test on the
Travis infrastructure but an initial before/after eyeballing seems to
confirm it is an improvement.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

.travis.yml

@@ -60,7 +60,7 @@ before_install:
 before_script:
   - ./configure ${CONFIG}
 script:
-  - make -j2 && ${TEST_CMD}
+  - make -j3 && ${TEST_CMD}
 matrix:
   include:
     # Sparse is GCC only