chore: cleanup lockfiles

- Delete package-lock.json. Bull currently has both yarn.lock and
package-lock.json files. When both are present, TravisCI uses yarn,
so in practice npm has not been used by CI. But the presence of a
package-lock.json file meant that contributors were not sure which
lock file, if any, needed to be updated. Yarn also logs a scary
warning if it detects a package-lock.json file.

- Cache yarn dependencies in TravisCI. This should make CI jobs
somewhat faster.

- Update yarn.lock file and ensure that it remains in sync with
package.json changes by using `--frozen-lockfile`.

.gitignore

@@ -2,4 +2,5 @@ node_modules
 tmp
 coverage
 *.rdb
-.vscode
+.vscode
+package-lock.json

.travis.yml

@@ -2,6 +2,9 @@ language: node_js
 
 sudo: false
 
+cache:
+  yarn: true
+
 # test on node.js versions
 node_js:
   - '12'
@@ -11,6 +14,15 @@ node_js:
 services:
   - redis-server
 
+before_install:
+  # Use a specific version of yarn in CI. This ensures yarn.lock format doesn't change.
+  - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.19.1
+  - export PATH="$HOME/.yarn/bin:$PATH"
+
+install:
+  # ensure unexpected changes to yarn.lock break the build
+  - yarn install --frozen-lockfile --non-interactive
+
 script:
   - npm run prettier -- --list-different
   - npm run test