ci: try to fix travis-ci build

Author: Brooooooklyn

.travis.yml

@@ -1,15 +1,18 @@
 language: python
-python:
-  - "3.5"
-  - "3.6"
-  - "3.7"
 os: osx
-osx_image: xcode9.3
+
+env:
+  - PYTHON_VERSION=3.5.6
+  - PYTHON_VERSION=3.6.6
+  - PYTHON_VERSION=3.7.0
+
 cache: cargo
 before_install:
   - brew update
-  - brew install rust
+  - brew install rust pyenv
   - rustup default nightly
+  - pyenv install $PYTHON_VERSION
+  - pyenv global $PYTHON_VERSION
 
 install:
   - pip install -r requirements-dev.txt