SSL.SSLv23_METHOD is broken on Ubuntu

https://bugs.launchpad.net/ubuntu/+source/openssl/+bug/861137
As a result all HTTPS based calls from Vumi will most likely fail on this platform with the error: 

```
'SSL routines', 'SSL23_GET_SERVER_HELLO',
'tlsv1 alert internal error'
```

this patch is a work around:

```diff
diff --git a/vumi/application/sandbox.py b/vumi/application/sandbox.py
index d845359..c76d8f5 100644
--- a/vumi/application/sandbox.py
+++ b/vumi/application/sandbox.py
@@ -20,7 +20,8 @@ from twisted.python.failure import Failure
 from twisted.web.client import WebClientContextFactory

 from OpenSSL.SSL import (
-    VERIFY_PEER, VERIFY_FAIL_IF_NO_PEER_CERT, VERIFY_CLIENT_ONCE, VERIFY_NONE)
+    VERIFY_PEER, VERIFY_FAIL_IF_NO_PEER_CERT, VERIFY_CLIENT_ONCE, VERIFY_NONE,
+    SSLv3_METHOD)

 from vumi.config import ConfigText, ConfigInt, ConfigList, ConfigDict
 from vumi.application.base import ApplicationWorker
@@ -708,6 +709,8 @@ class LoggingResource(SandboxResource):

 class HttpClientContextFactory(WebClientContextFactory):

+    method = SSLv3_METHOD
+
     def __init__(self, verify_options=None):
         self.verify_options = verify_options
 ```