travis: add a build using the synchronous name resolver

... since default uses the threaded one and we test the c-ares build
already.

Closes #2689

.travis.yml

@@ -75,6 +75,10 @@ matrix:
           compiler: clang
           dist: trusty
           env: T=debug C="--with-mbedtls --without-ssl"
+        - os: linux
+          compiler: clang
+          dist: trusty
+          env: T=debug C="--disable-threaded-resolver"
         - os: linux
           compiler: clang
           dist: trusty