travis: Remove explicit npm reinstall

This also aims to kick off a new PR build, as the previous commit
triggered a new push build with the new commit, but the new PR build was
still using the previous one:

Push: https://travis-ci.org/mbland/custom-links/builds/257941056
PR:   https://travis-ci.org/mbland/custom-links/builds/257941053

Of note is that the `if [[ ( "$TRAVIS_PULL_REQUEST" ...` bit appears in
the former's logs, but not in the latter's.

.travis.yml

@@ -15,7 +15,6 @@ before_install:
 - export DISPLAY=:99.0
 - sh -e /etc/init.d/xvfb start
 - export PATH=$PWD/node_modules/phantomjs-prebuilt/lib/phantom/bin:$PATH
-- npm install -g npm@^5.0.1
 - if [[ ( "$TRAVIS_PULL_REQUEST" != 'false' || "$TRAVIS_BRANCH" == 'master' ) && "$TRAVIS_JOB_NUMBER" =~ [.]1$ ]]; then curl -L https://codeclimate.com/downloads/test-reporter/test-reporter-latest-linux-amd64 > ./cc-test-reporter; chmod +x ./cc-test-reporter; fi
 script:
 - "./go setup"