Use latest pybind11 in CI

Related to conda-forge/xtensor-python-feedstock#56

Author: robertodr

.travis.yml

@@ -112,7 +112,7 @@ before_install:
       elif [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
         export CXX=clang++ CC=clang PYTHONHOME=$HOME/miniconda;
       fi
-    - PYBIND11_VERSION=${PYBIND11_VERSION:-2.2.4}
+    - PYBIND11_VERSION=${PYBIND11_VERSION:-2.4.2}
 
 install:
     # Define the version of miniconda to download