Don't load on older vim

Fix errors when first opening old versions of vim.

vim-lsp requires json_encode and lambda and so do we. Skip loading if
they're not available.

plugin/lsp_settings.vim

@@ -1,4 +1,4 @@
-if exists('g:loaded_lsp_settings')
+if exists('g:loaded_lsp_settings') || !exists('*json_encode') || !has('lambda')
   finish
 endif
 let g:loaded_lsp_settings= 1