Revert "Get rid of the fancy magic of preinstalling Python versions"

This reverts commit b561d1fc17.

.travis.yml

@@ -1,4 +1,5 @@
 language: python
+sudo: true
 python:
   - 2.7
   - 3.4
@@ -13,27 +14,37 @@ env:
   - JEDI_TEST_ENVIRONMENT=36
   - JEDI_TEST_ENVIRONMENT=37
 
+addons:
+  apt:
+    packages:
+     # Required to properly create a virtual environment with system Python 3.4.
+     - python3.4-venv
+
 matrix:
   allow_failures:
     - python: pypy
     - env: TOXENV=sith
-
   include:
     - python: 3.6
       env:
         - TOXENV=cov
         - JEDI_TEST_ENVIRONMENT=36
-
     - python: 3.6
       env: TOXENV=sith
-
     # For now ignore pypy, there are so many issues that we don't really need
     # to run it.
     #- python: pypy
-
     - python: "nightly"
       env:
         - JEDI_TEST_ENVIRONMENT=36
+before_install:
+    - ./travis_install.sh
+    # Need to add the path to the Python versions in the end. This might add
+    # something twice, but it doesn't really matter, because they are appended.
+    - export PATH=$PATH:/opt/python/3.5/bin
+    # 3.6 was not installed manually, but already is on the system. However
+    # it's not on path (unless 3.6 is selected).
+    - export PATH=$PATH:/opt/python/3.6/bin
 install:
     - pip install --quiet tox-travis
 script: