travis: add c-ares enabled builds linux + osx

Closes #1868

.travis.yml

@@ -25,6 +25,10 @@ matrix:
           compiler: gcc
           dist: trusty
           env: T=normal
+        - os: linux
+          compiler: gcc
+          dist: trusty
+          env: T=normal C=--enable-ares
         - os: linux
           compiler: clang
           dist: trusty
@@ -32,6 +36,9 @@ matrix:
         - os: osx
           compiler: gcc
           env: T=debug
+        - os: osx
+          compiler: gcc
+          env: T=debug C=--enable-ares
         - os: osx
           compiler: gcc
           env: T=debug C="--with-ssl=/usr/local/opt/openssl --with-libmetalink"