travis: install python3.5 for trusty in order to run pip3

.travis.yml

@@ -25,8 +25,9 @@ matrix:
 
 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 apt-get install -y clang cmake g++ gcc libeigen3-dev swig3.0 python-dev gcc-5 g++-5
+  - sudo apt-get install -y clang cmake g++ gcc libeigen3-dev swig3.0 python-dev gcc-5 g++-5 python3.5
   - eval "${MATRIX_EVAL}"
   - if [ -n "${RUN_PYTEST}" ]; then pip3 install -r ./EKF/tests/pytest/requirements.txt; fi