vimrc: update for newest vim-mkbuild plugin

The vim-mkbuild plugin has been improved.  It supports two diferent file
search sources for Unite!

* <leader>E maps to unite using the mkbuild/file source, which provides
  all files in the project, including other projects referenced
  externally in the optional extrefs file.

* <leader>e maps to unite using the mkbuild/file_xref source, which
  provides that subset of files used in a specific mkbuild build.  This
  version requires the use of <leader>x first to set a build.  If no
  build is available, mkbuild/file_xref instead works just like
  mkbuild/file.