Be explicit about which path we bundle into and cache it explicitly

Because this is a language: nodejs project, Travis doesn't automatically
infer that bundler stuff should be cached, or automatically append
caching parameters to the bundler invocations. This adds manual caching
for bundler.

Note: the default path for bundler caching on travis is vendor/bundle,
but this opts to use .vendor/bundle so that the files are ignored by
most tooling, like tslint and webpack configurators. It seems better to
ignore the whole folder using filesystem conventions instead of adding
exceptions to each tool looking at the filesystem.

.travis.yml

@@ -2,16 +2,16 @@ language: node_js
 node_js:
 - 8.9.4
 cache:
-  bundler: true
   yarn: true
   directories:
    - /home/travis/.rvm/
+   - /home/travis/bundle
 before_install:
   - rvm install 2.5.1
   - rvm use 2.5.1
 install:
   - yarn install
-  - bundle install --jobs=3 --retry=3
+  - bundle install --jobs=3 --retry=3 --path=/home/travis/bundle
 before_script:
 - sudo apt-get install curl -y
 - mv node_modules/.bin/which node_modules/.bin/which.backup