Updated brew_installs.sh script

* Fixed typo in the license URL...
+ Added updating Python2 and installing Python3.
+ Added installing Markdown and MultiMarkdown.
+~ Improved the MacVim installation. Will now replace file system's
   icons for the files MacVim recognizes. And it will now compile with
   support for Python3 scripts inside the plugins.
- Removed useless stuff from installation process (if you didn't have
  it installed of course).