Add xref check to travis

.travis.yml

@@ -6,4 +6,5 @@ otp_release:
   #- 18.3 not supported (string:lexemes/2, tricky binary comprehension, map typespec with `:=`, ?assertEqual/3)
 script:
   - ./rebar3 compile
+  - ./rebar3 xref
   - ./rebar3 eunit

rebar.config

@@ -6,6 +6,15 @@
         {lager, "3.6.3"}
         ]}.
 
+{xref_checks,
+ [undefined_function_calls,
+  undefined_functions,
+  locals_not_used,
+  %% exports_not_used,
+  deprecated_function_calls,
+  deprecated_functions
+  ]}.
+
 {relx, [{release, { mtp_proxy, "0.1.0" },
          [lager,
           ranch,