bump libressl version (#4062)

diff --git a/Jenkinsfile b/Jenkinsfile
index 48e737c..474f502 100644
--- a/Jenkinsfile
+++ b/Jenkinsfile
@@ -60,7 +60,7 @@
     ],
     [
         label: 'docker',
-        imageName: 'pyca/cryptography-runner-jessie-libressl:2.6.3',
+        imageName: 'pyca/cryptography-runner-jessie-libressl:2.6.4',
         toxenvs: ['py27'],
     ],
     [