.vimrc: Enable `g:solarized_termtrans`

It improves the display in OS X Terminal.app.

http://ethanschoonover.com/solarized/vim-colors-solarized

Ref. #425.
brew.sh: Use `--with-iri` for `wget`

The older `--enable-iri` flag is deprecated.

Closes #445.
.exports: Remove `export HOMEBREW_CASK_OPTS`

It’s not needed anymore now that the `Caskfile` has been removed.

Closes #446.
merging conflicts