Fix travis build on osx

.travis.yml

@@ -1,27 +1,43 @@
 # https://travis-ci.org/HearthSim/decrunch
-dist: xenial
-language: python
-
-os:
-  - linux
-  - osx
-
-python:
-  - 3.5
-  - 3.6
-  - 3.7
-
-cache: pip
-
-addons:
-  apt:
-    sources:
-      - ubuntu-toolchain-r-test
-    packages:
-      - g++-8
+matrix:
+  include:
+    - os: linux
+      dist: xenial
+      language: python
+      python:
+        - 3.5
+        - 3.6
+        - 3.7
+      cache: pip
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-8
+    - os: osx
+      language: generic
+      env:
+        matrix:
+          - PYTHON_VERSION=3.5.7
+          - PYTHON_VERSION=3.6.8
+          - PYTHON_VERSION=3.7.3
+      addons:
+        homebrew:
+          packages:
+            - pyenv
+          update: true
 
 env:
-  - CXX=g++-8
+  global:
+    - CXX=g++-8
+
+before_install:
+  - |
+  if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
+    pyenv install "$PYTHON_VERSION"
+    pyenv global "$PYTHON_VERSION"
+  fi
 
 install:
   - pip install --upgrade pip wheel setuptools