bump libressl to 2.6.2 (#3967)

diff --git a/Jenkinsfile b/Jenkinsfile
index 60195a6..a969f8a 100644
--- a/Jenkinsfile
+++ b/Jenkinsfile
@@ -60,7 +60,7 @@
     ],
     [
         label: 'docker',
-        imageName: 'pyca/cryptography-runner-jessie-libressl:2.6.1',
+        imageName: 'pyca/cryptography-runner-jessie-libressl:2.6.2',
         toxenvs: ['py27'],
     ],
     [