Make MINLPTests compatible with JuMP 0.22

[frapac]

Add a new compatible entry in Project.toml, in a way that MINLPTests is compatible both with JuMP 0.21 and JuMP 0.22.

Testing locally, both Knitro and MadNP pass the tests with JuMP 0.22 and MINLPTests.

We can not force MINLPTests to use JuMP 0.22, as Juniper is not compatible with that version yet.

[codecov]

Codecov Report

Merging #34 (704668b) into master (1eb648f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #34   +/-   ##
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  Coverage   95.70%   95.70%           
=======================================
  Files          93       93           
  Lines        1047     1047           
=======================================
  Hits         1002     1002           
  Misses         45       45           

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[odow]

I also bumped the Ipopt version