Fixed lookup for 'exact' file nodes

I noticed that the `Gruntfile.js` lookup wasn't working and generally we also use `gulpfile.js` in lowercase.
Merge branch 'file-nodes-exact-symbols' of https://github.com/johngeorgewright/vim-webdevicons into johngeorgewright-file-nodes-exact-symbols
Fixes png extension changes from johngeorgewright-file-nodes-exact-symbols
Merge branch 'johngeorgewright-file-nodes-exact-symbols'