changed markdown_preview plugin to hammer because author changed proj…

…ect name and rake was failing on it

Rakefile

@@ -129,7 +129,7 @@ vim_plugin_task "haml",             "git://github.com/tpope/vim-haml.git"
 vim_plugin_task "indent_object",    "git://github.com/michaeljsmith/vim-indent-object.git"
 vim_plugin_task "javascript",       "git://github.com/pangloss/vim-javascript.git"
 vim_plugin_task "jslint",           "git://github.com/hallettj/jslint.vim.git"
-vim_plugin_task "markdown_preview", "git://github.com/robgleeson/vim-markdown-preview.git"
+vim_plugin_task "hammer",           "git://github.com/robgleeson/hammer.vim.git"
 vim_plugin_task "nerdtree",         "git://github.com/wycats/nerdtree.git"
 vim_plugin_task "nerdcommenter",    "git://github.com/ddollar/nerdcommenter.git"
 vim_plugin_task "surround",         "git://github.com/tpope/vim-surround.git"