Jenkinsfile: Move to single quoted strings for shell steps.

Jenkinsfile

@@ -6,23 +6,23 @@ node {
          env.VERSION_STRING = GIT_TAG
      }
      docker.image('golang:1.8').inside {
-         stage "test"
+         stage 'test'
          sh 'env'
-         sh "curl https://glide.sh/get | sh"
-         sh "mv ${GOPATH}/src/github.com/UNINETT ${GOPATH}/src/github.com/uninett"
-         sh "cd ${GOPATH}/src/github.com/uninett/jwt-tokenissuer"
-         sh "glide install"
-         sh "make test"
+         sh 'curl https://glide.sh/get | sh'
+         sh 'mv $GOPATH/src/github.com/UNINETT ${GOPATH}/src/github.com/uninett'
+         sh 'cd $GOPATH/src/github.com/uninett/jwt-tokenissuer'
+         sh 'glide install'
+         sh 'make test'
 
-         stage "build"
-         sh "make build"
+         stage 'build'
+         sh 'make build'
          stash includes: 'dist/linux-amd64/jwt-tokenissuer', name: 'binary'
      }
      docker.image('cdrx/fpm-debian:8').inside {
-         stage "package"
+         stage 'package'
          unstash 'binary'
-         sh "mv jwt-tokenissuer /src/"
-         sh "fpm -s dir /src=/opt/jwt-tokenissuer -t deb -n jwt-tokenissuer -v ${VERSION_STRING}"
+         sh 'mv jwt-tokenissuer /src/'
+         sh 'fpm -s dir /src=/opt/jwt-tokenissuer -t deb -n jwt-tokenissuer -v ${VERSION_STRING}'
          archive includes: '*.deb'
      }
 }