start testing against alpine (#3724)

Drop jessie-libre 2.5.4 since alpine uses libre (boo)
diff --git a/Jenkinsfile b/Jenkinsfile
index 18091c7..5440093 100644
--- a/Jenkinsfile
+++ b/Jenkinsfile
@@ -55,11 +55,6 @@
     ],
     [
         label: 'docker',
-        imageName: 'pyca/cryptography-runner-jessie-libressl:2.5.4',
-        toxenvs: ['py27'],
-    ],
-    [
-        label: 'docker',
         imageName: 'pyca/cryptography-runner-ubuntu-xenial',
         toxenvs: ['py27', 'py35'],
     ],
@@ -80,6 +75,11 @@
         imageName: 'pyca/cryptography-runner-fedora',
         toxenvs: ['py27', 'py35'],
     ],
+    [
+        label: 'docker',
+        imageName: 'pyca/cryptography-runner-alpine:latest',
+        toxenvs: ['py36'],
+    ],
 ]
 
 /* Add the linkcheck job to our config list if we're on master */