[travis] Upgrade setuptools and pip

This code upgrades the setuptools and pip. This change
stems from the fix at:
chaoss/grimoirelab-perceval#643.
It was needed to fix the CI tests that were failing due
to the following error:

```
File "../python3.5/site-packages/setuptools/sandbox.py", line 44, in _execfile
  code = compile(script, filename, 'exec')
File "/tmp/easy_install-wu9lgsxc/pandoc-2.0a4/setup.py", line 11
  error = f"pip is not installed, refer to <{url}> for instructions."
                                                                    ^
SyntaxError: invalid syntax
```

Signed-off-by: Valerio Cosentino <[email protected]>

.travis.yml

@@ -7,6 +7,8 @@ python:
 sudo: false
 
 before_install:
+  - pip install --upgrade setuptools
+  - pip install --upgrade pip
   - pip install -r "requirements.txt"
   - pip install -r "requirements_tests.txt"
   - pip install flake8