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