fix

.travis.yml

@@ -55,7 +55,7 @@ matrix:
     - os: osx # minimum osx Xcode 8.3
       osximage: xcode8.3
       env:
-        PY=3.7
+        PY=system
         BUILD_PYTHON=OFF
         BUILD_NUMPY=OFF
         BUILD_SERIALIZATION=OFF
@@ -66,9 +66,7 @@ git:
 # Install packages (pre-installed: pytest numpy)
 install:
   - if [[ ${TRAVIS_OS_NAME} == "osx" ]]; then
-      brew update;
-      brew upgrade python;
-      pyenv rehash;
+      alias pip=pip2;
     fi
   - pyenv versions
   - pyenv global ${PY}