[fix] #54 fix gitlab ci to use pyenv python 3.6.4 already installed in the docker image

.gitlab-ci.yml

@@ -6,7 +6,7 @@ stages:
   - release
 
 variables:
-    PYENV_PYTHON_VERSION: 3.6.8
+    PYENV_PYTHON_VERSION: 3.6.4
 
 image: registry.duniter.org/docker/python3/duniterpy-builder:0.0.4
 
@@ -35,8 +35,6 @@ push_to_github:
     - export PYENV_ROOT="$HOME/.pyenv"
     - export PATH="$PYENV_ROOT/bin:$PATH"
     - eval "$(pyenv init -)"
-    # fixme: pre-install python version in docker image to gain speed
-    - pyenv install $PYENV_PYTHON_VERSION
     - pyenv shell $PYENV_PYTHON_VERSION
 
 .changes: &changes