Rename MathProg to JuMP

This pull request updates METADATA to reflect the renaming of MathProg to JuMP.

The package was initially called JuMP (Julia for Mathematical Programming) but was renamed to MathProg after considering some alternatives to follow the convention of naming packages with generic names related to functionality.

However, GNU MathProg already exists and we were receiving feedback as the package gained some notice in the OR community that it was confusing. Alternatives that are similarly explicit are either overly verbose or misleading/unnatural.

Finally, all other comparable software packages have 4 letter names, e.g. AMPL, PuLP (Python), GAMS.