Latest version seems to be broken (`variable 'X' is unbound`)

[asaaki]

I tried to install the plugin but it fails on compilation:

 ~/Development/rebarhextest> ./rebar3 plugins
===> Fetching rebar3_hex ({pkg,<<"rebar3_hex">>,<<"1.2.0">>})
===> Fetching jsx ({pkg,<<"jsx">>,<<"2.6.1">>})
===> Compiling jsx
===> Compiling rebar3_hex
===> Compiling /Users/chris/Development/rebarhextest/_build/default/plugins/rebar3_hex/src/rebar3_hex_tar.erl failed
/Users/chris/Development/rebarhextest/_build/default/plugins/rebar3_hex/src/rebar3_hex_tar.erl:33: variable 'X' is unbound

/Users/chris/Development/rebarhextest/_build/default/plugins/rebar3_hex/src/rebar3_hex_tar.erl:34: Warning: variable 'X' is unused
/Users/chris/Development/rebarhextest/_build/default/plugins/rebar3_hex/src/rebar3_hex_tar.erl:34: Warning: variable 'X' shadowed in generate

===> Plugin rebar3_hex not available. It will not be used.

plugins <task>:
  list           List local and global plugins for this project
  upgrade        Uprade plugins

(This is reproducible, my docker image builds also fail: https://hub.docker.com/r/asaaki/base-erlang/builds/bdixrwrcubozhw9squjqgn9/)

It seems this commit (705efd0) broke it.

[asaaki]

Thank you for the quick fix and release! :bow: