disable package-lock.json on Travis

Travis frontend builds run on a Linux platform, different from the Mac
platform typically used for development. This can cause differences in
the package-lock.json file due to the inclusion of optional modules.

See this lengthy npm thread that describes the problem:

npm/npm#17722

To avoid this issue, this change runs `npm config set package-lock
false` to disable the use of lock files when doing the npm install.

We already do something similar to this in Capital Framework:

https://github.com/cfpb/capital-framework/blob/821dd4fafb65b947ea436fd57b18a195f57e71b2/.travis.yml#L9

This change is needed because if the package-lock.json file is modified
by the build, then the Python wheel name generated by
setuptools-git-version is marked as dirty and won't match the latest
tag. This has bitten us before on other repositories.

.travis.yml

@@ -9,6 +9,7 @@ before_script:
   - npm install -g bower gulp-cli
   - bower install
 script:
+- npm config set package-lock false
 - tox
 - python2.7 setup.py bdist_wheel
 - ls dist/*