Update gitlab-ci configuration

.gitlab-ci.yml

-image: python:3.5
+image: python:3.6
 
 cache:
     paths:
         - .tox
 
 before_script:
-    - pip install -U tox
+    # Setup locale
+    - export LC_ALL=en_US.utf-8
+    - export LANG=en_US.utf-8
+    # Install pyenv
+    - apt-get update
+    - apt-get install -y make build-essential libssl-dev zlib1g-dev libbz2-dev libreadline-dev libsqlite3-dev wget curl llvm libncurses5-dev libncursesw5-dev xz-utils tk-dev
+    - git clone https://github.com/pyenv/pyenv.git ~/.pyenv
+    - export PYENV_ROOT="$HOME/.pyenv"
+    - export PATH="$PYENV_ROOT/bin:$PATH"
+    - eval "$(pyenv init -)"
+    # Install tox
+    - pip install tox
 
 tests:
     script: