gmt: update to 4.5.12

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 [gmt4](https://github.com/Homebrew/homebrew-versions/pull/290/files)), I would suggest that we at least update the current GMT formula to support the minor upgrade of GMT from GMT 4.5.9 to GMT 4.5.12.  This GMT upgrade moved the GMT 4.5.9 source file into the legacy directory (ftp://ftp.soest.hawaii.edu/gmt/legacy), breaking the current GMT formula.