Update pyOpenSSL requirement version to 0.15

[ken5scal]

This is pull request for #155

Following command was failing due to required version of pyOpenSSL.
Therefore, I updated it.

easy_install ./drozer-2.3.4-py2.7.egg 

OpenSSL/crypto/crl.c:6:23: error: static declaration of 'X509_REVOKED_dup' follows non-static declaration
static X509_REVOKED * X509_REVOKED_dup(X509_REVOKED *orig) {
                      ^
/usr/local/opt/openssl/include/openssl/x509.h:751:15: note: previous declaration is here
X509_REVOKED *X509_REVOKED_dup(X509_REVOKED *rev);
              ^

[HenryHoggard]

Closed: See #182