Travis: default to HARNESS_JOBS=4

We can run tests in parallel by setting the HARNESS_JOBS environment
variable.
Reviewed-by: NDavid von Oheimb <david.von.oheimb@siemens.com>
(Merged from https://github.com/openssl/openssl/pull/12326)

.travis.yml

@@ -173,6 +173,7 @@ before_script:
           fi;
       fi
     - ./configdata.pm --dump
+    - export HARNESS_JOBS=${HARNESS_JOBS:-4}
     - cd $top
 
 script: