Rakefile: remove vim-specific modeline
Avoid taking sides or showing preference for particular text
editors. Text editors are _very_ personal preferences, and
explicitly supporting/acknowledging one would set precedence for
explicitly supporting/acknowledging others. This would lead
down the slippery slope of having modelines for every single
text editor in every single file we have.
For this same reason, we do not (and never will) expose
editor-specific suffixes in .gitignore.
Full disclosure: I'm a vim user myself