Remove port hardcoding in pact verifier http handler (#30)

# Description

Please include a summary of the change and which issue is fixed. Please
also include relevant motivation and context. List any dependencies that
are required for this change.

Fixes # (issue)

# Checklist:

- [x] I have ensured that all new code is covered by tests
- [x] I have run `make format` to format the code according to
guidelines
- [x] I have documented any new functions introduced
- [x] I have updated the changelog in accordance with
https://keepachangelog.com/en/1.1.0/
- [x] I have updated the README, if required

src/pact_verifier.erl

@@ -90,7 +90,7 @@ start_verifier(Provider, ProviderOpts) ->
                 Port1 = maps:get(port, ProviderOpts),
                 {Port1, undefined};
             <<"message">> ->
-                {ok, Port1, HttpPid1} = pact_verifier:start(8181, Provider),
+                {ok, Port1, HttpPid1} = pact_verifier:start(0, Provider),
                 {Port1, HttpPid1}
         end,
     gen_server:start(