Wrap stage tasks in a block

This fixes a warning from Jenkins that using `stage` without specifying
a block is deprecated.

Jenkinsfile

@@ -10,20 +10,22 @@ node {
       returnStdout: true
     ).trim()
 
-    stage 'Build'
+    stage('Build') {
         nodeEnv.inside("-e HOME=${workspace}") {
             sh "echo Building Hypothesis client \"${pkgVersion}\""
             sh 'make clean'
             sh 'make'
         }
+    }
 
-    stage 'Test'
+    stage('Test') {
         nodeEnv.inside("-e HOME=${workspace}") {
             sh 'make test'
         }
+    }
 
     if (isTag()) {
-        stage 'Publish'
+        stage('Publish') {
             nodeEnv.inside("-e HOME=${workspace}") {
                 withCredentials([
                     [$class: 'StringBinding', credentialsId: 'npm-token', variable: 'NPM_TOKEN']]) {
@@ -42,6 +44,7 @@ node {
                     sh "docker logs --follow ${c.id}"
                   }
         }
+    }
 }
 
 boolean isTag() {