Update README.md

Add homebrew steps for resolving the `gfortran` and `f2c` dependencies required by the `linalg` gem.
Update linalg dependency matching via homebrew.

 * `gfortran` is now installed as [part of the gcc formula](https://github.com/Homebrew/homebrew-science/issues/973#issuecomment-46328125).
 * `f2c` is part of `homebrew/headonly`.
Merge pull request #2 from mariusbutuc/patch-1

Update README.md