Problem: not all cores used on Travis

Solution: As Travis enables 2 cores let's use them both

ci_build.sh

@@ -5,6 +5,8 @@ set -e
 
 LIBZMQ=${PWD}/libzmq-build
 CPPZMQ=${PWD}/cppzmq-build
+# Travis machines have 2 cores
+JOBS=2
 
 if [ "$DRAFT" = "1" ] ; then
     # if we enable drafts during the libzmq cmake build, the pkgconfig
@@ -21,7 +23,7 @@ install_zeromq() {
                                               -DZMQ_BUILD_TESTS=OFF \
                                               -DCMAKE_BUILD_TYPE=Release \
                                               ${ZEROMQ_CMAKE_FLAGS}
-    cmake --build ${LIBZMQ}
+    cmake --build ${LIBZMQ} -- -j${JOBS}
 }
 
 # build zeromq first
@@ -31,13 +33,13 @@ if [ "${ZMQ_VERSION}" != "" ] ; then install_zeromq ; fi
 # build cppzmq
 pushd .
 ZeroMQ_DIR=${LIBZMQ} cmake -H. -B${CPPZMQ} ${ZEROMQ_CMAKE_FLAGS}
-cmake --build ${CPPZMQ}
+cmake --build ${CPPZMQ} -- -j${JOBS}
 cd ${CPPZMQ}
-ctest -V
+ctest -V -j${JOBS}
 popd
 
 # build cppzmq demo
 ZeroMQ_DIR=${LIBZMQ} cppzmq_DIR=${CPPZMQ} cmake -Hdemo -Bdemo/build
 cmake --build demo/build
 cd demo/build
-ctest
+ctest -V