New formula: gmt4

Since `gmt 5` is out and not entirely compatible with previous versions, the old formula is moved here and renamed `gmt4`. See https://github.com/Homebrew/homebrew-science/pull/473.

The two formulas are marked as incompatible.

I also updated `gmt4` version to `4.5.11`. 

~~Please merge this PR only after https://github.com/Homebrew/homebrew-science/pull/473 is merged.~~

Nope, this has to be merged **before** https://github.com/Homebrew/homebrew-science/pull/473 !