Fix memory leak in _X509_REVOKED_dup

The call to X509_REVOKED_new() will create two empty ASN1 structures
that were never freed, but simply replaced by a copy.
When doing multiple calls to CRL.get_revoked() on a big CRL, this
results in a huge memory leak.
This change adds two calls to free those empty ASN1 structures before
replacing them with the copy.

This change requires https://github.com/pyca/cryptography/pull/830 in
order to work.
Changelog entry
Add a leakcheck test for CRL.get_revoked
Make sure that lines are not longer than 80 characters
Remove unneeded calls

A serial number and a revocation date do not need to be set for the
leak to occur in the current version of pyOpenSSL.
Add a leakcheck test for CRL.add_revoked
Add a leakcheck test for _X509_REVOKED_dup
Require cryptography 0.3

Require cryptography 0.3 to have the ASN1_TIME_free binding.
Changelog entries
Merge pull request #74 from jgiannuzzi/fix_crl_memory_leak

Fix a memory leak in the CRL class's handling of the serial number and revocation date.