Try a bit more if modifying the PATH is now possible.

.travis.yml

@@ -27,9 +27,17 @@ matrix:
       env: TOXENV=sith
     - python: pypy
     - python: "3.7-dev"
+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=$PATH:/opt/python/3.3/bin
+    - export PATH=$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
-    - ./travis_install.sh
 script:
     - tox
 after_script:

travis_install.sh

@@ -9,9 +9,6 @@ if [[ $JEDI_TEST_ENVIRONMENT == "35" ]]; then
     VERSION=3.5
     DOWNLOAD=1
 fi
-if [[ $JEDI_TEST_ENVIRONMENT == "36" ]]; then
-    VERSION=3.6
-fi
 
 if [[ -z $VERSION ]]; then
     echo "Environments should already be installed"
@@ -30,5 +27,4 @@ if [[ $DOWNLOAD == 1 ]]; then
     sudo tar xjf $DOWNLOAD_NAME.tar.bz2 --directory /
 fi
 
-# Need to add the path in the end.
-export PATH=$PATH:/opt/python/$VERSION/bin
+echo "Successfully installed environment."