Update GMT

Whilst we wait for the resolution of [pull request #473: GMT 5.1.0](https://github.com/Homebrew/homebrew-science/pull/473) (which will require this formula to be renamed to [gtm4](https://github.com/Homebrew/homebrew-versions/pull/290/files)), I would suggest that we at least update the current GMT pull request to support the minor upgrade of GMT from GMT 4.5.9 to GMT 4.5.12.  This GMT update moved the GMT 4.5.9 source file into the [legacy](ftp://ftp.soest.hawaii.edu/gmt/legacy/) directory, breaking the current GMT formula.