Updated hashing to SHA256.

Updated the gem to sign with SHA256 instead of SHA1 and replaced MD5 with SHA256 as well.

Using SHA1 for signing api requests is a bad security practice, as well as using MD5. SHA1 is vulnerable to replay attacks where an attacker knows the body and signature for a valid API call because of the way the SHA1 algorithm works, the attacker can append null bytes to fill out the last iteration of the hashing algorithm, then append a malicious message. Because the attacker knows the hash value that will be used to sign the original+null_bytes+malicious_message, he can sign the new message without knowing the secret. To read more about length expansion attacks on SHA1, see https://blog.whitehatsec.com/hash-length-extension-attacks/.