Force Python 3 tox on Travis CI

pip may point to Python 2 but setup.py requires Python 3. Tox sdist may
fail when tox is installed for Python 2 on Travis CI.

https://github.com/python-ldap/python-ldap/pull/362

Signed-off-by: Christian Heimes <cheimes@redhat.com>

.travis.yml

@@ -68,7 +68,7 @@ env:
         - TOX_TESTENV_PASSENV="CFLAGS CI WITH_GCOV"
 
 install:
-  - pip install "pip>=7.1.0"
-  - pip install tox-travis tox codecov
+  - python3 -m pip install "pip>=7.1.0"
+  - python3 -m pip install tox-travis tox codecov
 
-script: CFLAGS="$CFLAGS_warnings $CFLAGS_std" tox
+script: CFLAGS="$CFLAGS_warnings $CFLAGS_std" python3 -m tox