Move npm to yarn

.travis.yml

@@ -4,11 +4,10 @@ group: beta
 language: node_js
 node_js:
   - "6"
-before_install:
-  - npm install truffle@3.2.2 -g
-  - npm i -g ethereumjs-testrpc
+cache:
+  yarn: true
 script:
   - testrpc > /dev/null &
   - truffle test
 after_script:
-  - npm run coveralls
+  - yarn run coveralls

README.md

@@ -24,6 +24,9 @@ truffle init
 To install the OpenZeppelin library, run:
 ```sh
 npm install zeppelin-solidity
+
+# If you are using yarn, add dependency like this -
+yarn add zeppelin-solidity
 ```
 
 After that, you'll get all the library's contracts in the `node_modules/zeppelin-solidity/contracts` folder. You can use the contracts in the library like so:

scripts/coveralls.sh

 #! /bin/bash
 
-npm run coverage && cat coverage/lcov.info | ./node_modules/coveralls/bin/coveralls.js
+yarn run coverage && cat coverage/lcov.info | ./node_modules/coveralls/bin/coveralls.js