[Easy] move to yarn (#20)

We are using yarn everywhere else and have deemed it superior over npm (faster, more intuitive CLI). Also, the failed dependabot update is not causing an error under yarn.

### Test Plan

CI

.gitignore

@@ -2,6 +2,7 @@ build/*
 coverage/*
 coverage.json
 node_modules/*
+package-lock.json
 
 .idea/
 .vscode/

.travis.yml

@@ -1,15 +1,15 @@
 language: node_js
 node_js:
-  - '10'
-cache: npm
+  - "10"
+cache: yarn
 install:
-  - npm install -g ganache-cli
-  - npm install
+  - yarn global add ganache-cli
+  - yarn --frozen-lockfile
 before_script:
   - ganache-cli > /dev/null &
 script:
-  - npm run lint
-  - npm run solhint
-  - npm test
+  - yarn lint
+  - yarn solhint
+  - yarn test
   - npx solium -d contracts/
-  - npm run coverage && cat ./coverage/lcov.info | coveralls
+  - yarn coverage && cat ./coverage/lcov.info | coveralls