Add .nvmrc to gitignore

As there are different node/npm versions in the wild it is common to use
virtual envs for development. To make it easy to use this with different
develompment enviroments it is a good idea to place a `.nvmrc` file in
your dev root. So a simple `nvm install` or `nvm use` do nvm magic with
the given version.

.gitignore

@@ -52,3 +52,4 @@ npm-debug.log
 .yo-rc.json
 .vendor/
 .solargraph.yml
+.nvmrc