Update Vundle syntax to reflect interface changes

According to the Vundle README, they are undergoing an interface change,
which means the 'Bundle' syntax is replaced with the 'Plugin'
syntax.[1][ref]

[ref]:
https://github.com/gmarik/vundle/blob/master/doc/vundle.txt#L345-L369

Issue: #5
Delete unused/unnecessary gitmodules

@necolas used vim-pathogen which requires/uses submodules as its
implementation style. Vundle doesn't use submodules, so removing all of
the submodules except vundle.

Issue: #5