.travis.tml: Remove before_install

These packages are not needed to run tests,
and they polute the testbed.

Author: jayvdb

.travis.yml

@@ -10,9 +10,6 @@ python:
   - 3.7
   - 3.8
 
-before_install:
-  - pip install -r requirements-dev.txt
-
 install:
   - python setup.py sdist && version=$(python setup.py --version) && pushd dist && pip install stdlib-list-${version}.tar.gz && popd