[git] Remove `[color] ui = true` from `gitconfig`

In order for Git to use color for all output written to the terminal,
the configuration variable `color.ui` needs to be set to `true` or
`auto`.

Starting with Git v1.8.4, `color.ui` defaults to `auto`, so setting
this value is no longer necessary.

References:

 * http://git-blame.blogspot.no/2013/08/git-184.html
 * https://www.kernel.org/pub/software/scm/git/docs/git-config.html
[vim] Use system clipboard as default register

References:

* http://vim.wikia.com/wiki/Accessing_the_system_clipboard
* http://vimcasts.org/episodes/accessing-the-system-clipboard-from-vim/
[vim] Make consistency changes
[git] Disable global line-ending normalization

Normalizing line-endings globally can be problematic as it prevents
some applications (e.g.: Homebrew) from updating if they contain files
with line-endings that get automatically normalize.

Ref: mathiasbynens/dotfiles#149.