travis: use pyenv in order to force pip3 version

.travis.yml

@@ -27,7 +27,9 @@ before_install:
   - sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
   - sudo add-apt-repository ppa:deadsnakes/ppa -y # ppa with latest Python versions to Ubuntu 14.04
   - sudo apt-get -qq update
+  - sudo pip install --egg pyenv
   - sudo apt-get install -y clang cmake g++ gcc libeigen3-dev swig3.0 python-dev gcc-5 g++-5 python3.5
+  - pyenv shell 3.5
   - eval "${MATRIX_EVAL}"
   - if [ -n "${RUN_PYTEST}" ]; then pip3 install -r ./EKF/tests/pytest/requirements.txt; fi