Emacs: Change key bindings for git-gutter+ mode

Currently, git-gutter+ key bindings are based on C-x xxx. These will
conflicts with key bindings narrow-to-xxx commands.

This patch move key binding of git-gutter+ to C-c xxx to incorporate
narrow-to-xxx.

,----[ Reconfigured Keybindings ]
| "C-c p"   -> git-gutter+-previous-hunk
| "C-c n"   -> git-gutter+-next-hunk
| "C-c v r" -> git-gutter+-revert-hunk
| "C-c v t" -> git-gutter+-show-hunk
`----

Tested-by: Ratheesh S <ratheeshreddy@gmail.com>
Signed-off-by: Ratheesh S <ratheeshreddy@gmail.com>