Fix make_ssl_certificate on Python 2

Fixes the following exception:

Traceback (most recent call last):

File "/home/travis/build/zoni/err/errbot/backends/base.py", line 520, in
_execute_and_send

for reply in replies:

    File "/home/travis/build/zoni/err/errbot/builtins/webserver.py",
    line 185, in generate_certificate

    make_ssl_certificate(key_path=key_path, cert_path=cert_path)

    File "/home/travis/build/zoni/err/errbot/builtins/webserver.py",
    line 58, in make_ssl_certificate

    cert.sign(pkey, u'sha256')

    File "build/bdist.linux-x86_64/egg/OpenSSL/crypto.py", line 827, in
    sign

    evp_md = _lib.EVP_get_digestbyname(_byte_string(digest))

    TypeError: initializer for ctype 'char *' must be a str or list or
    tuple, not unicode

errbot/builtins/webserver.py

@@ -56,7 +56,7 @@ def make_ssl_certificate(key_path, cert_path):
     pkey = crypto.PKey()
     pkey.generate_key(crypto.TYPE_RSA, 4096)
     cert.set_pubkey(pkey)
-    cert.sign(pkey, 'sha256')
+    cert.sign(pkey, 'sha256' if PY3 else b'sha256')
 
     f = open(cert_path, 'w')
     f.write(crypto.dump_certificate(crypto.FILETYPE_PEM, cert).decode('utf-8'))