Use anonymous GitHub API when GITHUB_TOKEN is not set

Signed-off-by: Alexei Barantsev <barancev@gmail.com>

scripts/travis/install.sh

@@ -7,7 +7,10 @@ if [[ ! -z $CHROME ]]; then
 fi
 
 if [[ ! -z $MARIONETTE ]]; then
-  export GECKODRIVER_DOWNLOAD=`curl -H "Authorization: token $GITHUB_TOKEN" -s 'https://api.github.com/repos/mozilla/geckodriver/releases/latest' | python -c "import sys, json; r = json.load(sys.stdin); print([a for a in r['assets'] if 'linux64' in a['name']][0]['browser_download_url']);"`
+  export GITHUB_AUTH="anonymous"
+  if [[ ! -z $GITHUB_TOKEN ]]; then
+    export GITHUB_AUTH="token $GITHUB_TOKEN"
+  export GECKODRIVER_DOWNLOAD=`curl -H "Authorization: $GITHUB_AUTH" -s 'https://api.github.com/repos/mozilla/geckodriver/releases/latest' | python -c "import sys, json; r = json.load(sys.stdin); print([a for a in r['assets'] if 'linux64' in a['name']][0]['browser_download_url']);"`
   curl -L -o geckodriver.tar.gz $GECKODRIVER_DOWNLOAD
   gunzip -c geckodriver.tar.gz | tar xopf -
   chmod +x geckodriver && sudo mv geckodriver /usr/local/bin