.travis.yml: don't run make check with multiple jobs

Let's challenge the convention that doing more at a time helps. It
certainly doesn't tell you unambiguously where in the test cycle you
were before the test hangs and exceeds the job time limit.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

.travis.yml

@@ -77,7 +77,7 @@ env:
     - SRC_DIR="."
     - BUILD_DIR="."
     - BASE_CONFIG="--disable-docs --disable-tools"
-    - TEST_CMD="make check -j3 V=1"
+    - TEST_CMD="make check V=1"
     # This is broadly a list of "mainline" softmmu targets which have support across the major distros
     - MAIN_SOFTMMU_TARGETS="aarch64-softmmu,arm-softmmu,i386-softmmu,mips-softmmu,mips64-softmmu,ppc64-softmmu,riscv64-softmmu,s390x-softmmu,x86_64-softmmu"
     - CCACHE_SLOPPINESS="include_file_ctime,include_file_mtime"