0.15.0

• Fix listen port / connect race-condition (#170)
• Require minimal OTP version 18 (#174)
• Add dialyzer to travis; some xref checks stubs
• More fine-grained typespecs in smtp_client and mimemail (#175)
• Add smtp client error types (#175)
• Fix typespec inet:socket_address() doesn't exist on OTP18 (#178)
• chore(Compiler Warning): Change from crypto:rand_uniform/2 to rand:uniform/1. (#172)