Switch from npm to yarn for package management

Yarn provides much faster package installation, better resilience to npm
registry connection issues and most importantly for us, a much better
lockfile format.

This commit:

1. Replaces the npm shrinkwrap with the yarn lockfile using the
   following steps:

   1. Checkout latest version of master

   2. Remove node_modules folder and re-run `npm install`

   3. Run `yarn import` to generate a lockfile

2. Modifies the Makefile, Jenkinsfile, package.json and .travis.yml
   scripts to run `yarn` instead of `npm`.

.travis.yml

@@ -1,7 +1,5 @@
 language: node_js
-cache:
-  directories:
-    - node_modules
+cache: yarn
 notifications:
   slack:
     rooms:
@@ -12,8 +10,8 @@ matrix:
     # See https://github.com/hypothesis/client/pull/27#discussion_r70611726
     - env: ACTION=lint
       node_js: '6.2'
-      script: npm run lint
+      script: yarn run lint
     - env: ACTION=test
       node_js: '6.2'
       after_success:
-        npm run report-coverage
+        yarn run report-coverage

Jenkinsfile

@@ -3,7 +3,7 @@
 node {
     checkout scm
 
-    nodeEnv = docker.image("node:6.2")
+    nodeEnv = docker.image("kkarczmarczyk/node-yarn:7.5")
     workspace = pwd()
 
     stage 'Build'
@@ -24,7 +24,7 @@ node {
                 [$class: 'StringBinding', credentialsId: 'npm-token', variable: 'NPM_TOKEN']]) {
 
                 sh "echo '//registry.npmjs.org/:_authToken=${env.NPM_TOKEN}' >> \$HOME/.npmrc"
-                sh "npm publish"
+                sh "yarn publish"
             }
         }
 

Makefile

@@ -13,11 +13,11 @@ clean:
 ## Run test suite
 .PHONY: test
 test: node_modules/.uptodate
-	npm test
+	yarn test
 
 .PHONY: lint
 lint: node_modules/.uptodate
-	npm run lint
+	yarn run lint
 
 .PHONY: docs
 docs:
@@ -26,8 +26,8 @@ docs:
 ################################################################################
 
 build/manifest.json: node_modules/.uptodate
-	npm run-script build
+	yarn run build
 
-node_modules/.uptodate: package.json npm-shrinkwrap.json
-	npm run-script deps 2>/dev/null || npm install
+node_modules/.uptodate: package.json yarn.lock
+	yarn run deps 2>/dev/null || yarn install
 	@touch $@