Use C++11 when testing with GCC 4.9

.travis.yml

@@ -69,7 +69,7 @@ matrix:
           packages:
             - g++-4.9
       env:
-        - MATRIX_EVAL="CC=gcc-4.9 && CXX=g++-4.9"
+        - MATRIX_EVAL="CC=gcc-4.9 && CXX=g++-4.9" CMAKE_CPP_STD=-DCMAKE_CXX_STANDARD=11
 
     # GCC 7, draft enabled, latest libzmq (default)
     - os: linux

ci_build.sh

@@ -39,7 +39,8 @@ cppzmq_build() {
     pushd .
     CMAKE_PREFIX_PATH=${LIBZMQ} \
     cmake -H. -B${CPPZMQ} -DENABLE_DRAFTS=${ENABLE_DRAFTS} \
-                          -DCOVERAGE=${COVERAGE}
+                          -DCOVERAGE=${COVERAGE} \
+                          ${CMAKE_CPP_STD:-}
     cmake --build ${CPPZMQ} -- -j${JOBS}
     popd
 }