Gracefully handle required missing retire arguments

[starbelly]

Closes #253

• Add helper function in rebar3_hex_retire that gets a required param or raises
• add basic tests for retire in integration suite
• add post retirement route support in hex api model

[oubiwann]

Any chance of a point release with this fix in it? :-D

[starbelly]

@oubiwann Yup, v6.11.6 released.