bump libressl version (#3920)

diff --git a/Jenkinsfile b/Jenkinsfile
index 93819ab..8742e85 100644
--- a/Jenkinsfile
+++ b/Jenkinsfile
@@ -60,7 +60,7 @@
     ],
     [
         label: 'docker',
-        imageName: 'pyca/cryptography-runner-jessie-libressl:2.6.0',
+        imageName: 'pyca/cryptography-runner-jessie-libressl:2.6.1',
         toxenvs: ['py27'],
     ],
     [