Enforce bower 1.7.9 to avoid bower/bower#2391

.travis.yml

@@ -14,7 +14,7 @@ cache:
 before_install:
 - "if [ -d node_modules ] && [ x$(cat node_modules/.last-node-version 2>/dev/null) != x$(node -e 'console.log(process.version)') ]; then npm rebuild && node -e 'console.log(process.version)' > node_modules/.last-node-version; fi"
 before_script:
-- npm install web-component-tester bower polylint
+- npm install web-component-tester bower@1.7.9 polylint
 - $(npm bin)/bower install
 - $(npm bin)/polylint --no-recursion
 script: