requirements: Fix dependencies for cryptography.

Apparently the module "cryptography" (used by service_identity) has some
dependencies that weren't initially included in requirements/scrapy.txt.

Python 2 and 3: six, cffi, asn1crypto, idna
Python 2 only: enum34, ipaddress

Out of these modules, the ones that were already included in
requirements/common.txt have been removed, resulting in:

Python 2 and 3: asn1crypto
Python 2 only: ipaddress

Credits as well to @adnrs96, whose help was essential to catch this issue.

requirements/scrapy.txt

@@ -1,5 +1,11 @@
 Scrapy==1.4.0
 cryptography==1.9
+asn1crypto==0.22.0
+
+# Required by cryptography, only for Python 2 (should be removed once we
+# migrate to Python 3):
+ipaddress==1.0.18
+
 parsel==1.2.0
 pyOpenSSL==17.0.0
 w3lib==1.17.0

version.py

@@ -1,2 +1,2 @@
 ZULIP_VERSION = "1.6.0+git"
-PROVISION_VERSION = '5.9'
+PROVISION_VERSION = '5.10'