fix(CI): Update CircleCI and Travis-CI build configs (#121)

* chore: Update CircleCI's Node image to 12.15

* fix: Remove global Yarn install from CircleCI

Yarn is automatically installed globally in the Node images

@see https://circleci.com/docs/2.0/yarn/#using-yarn-in-circleci

* fix: Remove global install of Yarn in Travis-CI

Yarn is provided automatically in the `node_js` build

* fix: Change CircleCI from 12.15 to lts

12.15 doesn't exist in the Docker repository

.circleci/config.yml

@@ -2,7 +2,7 @@ version: 2
 jobs:
   build:
     docker:
-      - image: circleci/node:10.13
+      - image: circleci/node:lts
 
     working_directory: ~/repo
 
@@ -11,9 +11,6 @@ jobs:
       - run:
           name: update-npm
           command: 'sudo npm i -g npm@latest'
-      - run:
-          name: install yarn
-          command: 'sudo npm i -g yarn@latest'
 
       # Download and cache dependencies
       - restore_cache:

.travis.yml

@@ -8,8 +8,6 @@ node_js:
 before_install:
   # Even though we support Node 6, we still want to use NPM 6
   - npmv=$(echo $(npm -v) | head -c 1); if [ "$npmv" -lt "6" ]; then npm i -g npm; fi
-  # We install Yarn to run unit tests
-  - npm i -g yarn@latest
 
 install:
   - npm ci