PropEr has a GPLv3 license

[FairPlayer4]

The GPLv3 license is not compatible with the Apache 2.0 license of this project.
I don't know if this really is a real legal issue but some might consider this a problem.

I saw in the antidote_crdt PR AntidoteDB/antidote_crdt#14 that the proper dependency was removed because of this issue.
In the antidote_crdt PR AntidoteDB/antidote_crdt#39 I created a somewhat elegant solution for this problem.

Additionally, the plugins in the rebar.config can be changed to project_plugins to avoid compilation of these plugin in projects that use riak_core_lite. (e.g. in antidote the plugins rebar3_format and rebar3_proper get compiled even though they are not needed)

[albsch]

See License issue thread.

[albsch]

Moving proper to project_plugins and using the proper dependency only in the test profile solves the issue.