travis: add no-TCG build

It's fairly easy for --disable-tcg to bitrot.  Test it in our CI.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-Id: <20170714093016.10897-1-pbonzini@redhat.com>
Signed-off-by: NFam Zheng <famz@redhat.com>

.travis.yml

@@ -86,6 +86,9 @@ matrix:
     - env: CONFIG="--enable-trace-backends=ust"
            TEST_CMD=""
       compiler: gcc
+    - env: CONFIG="--disable-tcg"
+           TEST_CMD=""
+      compiler: gcc
     - env: CONFIG=""
       os: osx
       compiler: clang