.vimrc update

According to the recommended commit suggestion as mentioned
https://github.com/torvalds/linux/pull/17, this commit has full
description of commit message.

In this commit, I added following configuration in my .vimrc:
  - syntax on: for default syntax code highlight for gnome-terminal
  - emulator color check for 256 colors

Since I used to share my vim-config on several machine,
it is nice to have same configurations.