add test cases for malformed/invalid base64 and version byte

[tmaher]

• add a test against an invalid version (any value othe than 0x80, say 0x00 and 0x81)
• add a test vector that contains invalid base64 encodings, e.g. whitespace, line endings, missing padding characters, last bits not set to zero, / or + character etc.

[hgmnz]

Thanks @tmaher. Let me know what you think: https://github.com/fernet/fernet-rb/pull/31

[hgmnz]

Fixed in https://github.com/fernet/fernet-rb/commit/d05eab8b0851773539f9ed63bfafec91161702dc and https://github.com/fernet/fernet-rb/commit/65cfcf86b512e2a4ad46b156bb22355445528b3e