ci: Travis: use install

Fixes a warning/msg:

> Could not locate requirements.txt. Override the install: key in your
> .travis.yml to install dependencies.

.travis.yml

@@ -20,7 +20,7 @@ python:
   - 3.5
   - 3.6
   - 3.7
-before_install:
+install:
   - if [ $CI_TARGET = tests ]; then
       eval "$(curl -Ss https://raw.githubusercontent.com/neovim/bot-ci/master/scripts/travis-setup.sh) nightly-x64";
       pip install -q scrutinizer-ocular tox-travis;