Use _.indexOf instead of _.contains

_.indexOf supports a isSorted flag which can be used
since the extension arrays are pre-sorted.

src/stdlib/fs.coffee

@@ -152,35 +152,33 @@ module.exports =
     undefined
 
   isCompressedExtension: (ext) ->
-    _.contains([
+    _.indexOf([
       '.gz'
       '.jar'
       '.tar'
       '.zip'
-    ], ext)
+    ], ext, true) >= 0
 
   isImageExtension: (ext) ->
-    _.contains([
+    _.indexOf([
       '.gif'
       '.jpeg'
       '.jpg'
       '.png'
       '.tiff'
-    ], ext)
+    ], ext, true) >= 0
 
   isPdfExtension: (ext) ->
-    _.contains([
-      '.pdf'
-    ], ext)
+    ext is '.pdf'
 
   isMarkdownExtension: (ext) ->
-    _.contains([
+    _.indexOf([
       '.markdown'
       '.md'
       '.mkd'
       '.mkdown'
       '.ron'
-    ], ext)
+    ], ext, true) >= 0
 
   readObject: (path) ->
     contents = @read(path)