adding yarn lock file and removing it from git ignore (webpack#3648)

* adding yarn lock file and removing it from git ignore
* travis will now run yarn by default that it sees the lock file

.gitignore

@@ -9,5 +9,4 @@
 .DS_Store
 *.log
 .idea
-yarn.lock
 .vscode

.travis.yml

@@ -36,7 +36,7 @@ matrix:
 script: npm run travis:$JOB_PART
 
 before_install: if [[ `npm -v` != 3* ]]; then npm i -g npm@3; fi
-install:
+before_script:
   - bash ./ci/travis-install.sh
 after_success:
   - cat ./coverage/lcov.info | node_modules/.bin/coveralls --verbose

ci/travis-install.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
 set -ev
 
-npm install yarn -g && yarn install && yarn link || true && yarn link webpack;
+yarn link || true && yarn link webpack;