Update .travis.yml

disable parallel build

.travis.yml

@@ -1,7 +1,7 @@
 language: cpp
 dist: bionic
 os: linux
-compiler: clang
+compiler: gcc
 addons:
     apt:
         sources:
@@ -20,7 +20,7 @@ install:
   - export PATH=$VERILATOR_ROOT/bin:$PATH 
 
 script:   
-  - make -j
+  - make
   - ci/test_runtime.sh
   - ci/test_driver.sh
   - ci/test_riscv_isa.sh