feat: remove yarn in favor of npm

.node-version

+v8.9.1

.travis.yml

@@ -3,14 +3,15 @@ sudo: required
 group: beta
 language: node_js
 node_js:
-  - "6"
+  - "8"
 cache:
-  yarn: true
+  directories:
+    - node_modules
 before_script:
   - truffle version
-  - yarn list
+  - npm list
 script:
-  - yarn test
+  - npm test
 notifications:
   slack:
     rooms:

README.md

@@ -25,9 +25,6 @@ To install the OpenZeppelin library, run:
 ```sh
 npm init
 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:

docs/source/getting-started.rst

@@ -12,10 +12,6 @@ To install the OpenZeppelin library, run::
 	npm init # follow instructions
 	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::
 
 	import "zeppelin-solidity/contracts/ownership/Ownable.sol";

package.json

@@ -19,6 +19,9 @@
     "security",
     "zeppelin"
   ],
+  "engines": {
+    "node": ">=8.9.1"
+  },
   "author": "Manuel Araoz <manuelaraoz@gmail.com>",
   "license": "MIT",
   "bugs": {