Mock out process.platform as darwin by default

This avoids calling the non-existent getModifierState method on our fake keyboard events when running on Travis.

Author: Nathan Sobo

spec/keymap-manager-spec.coffee

@@ -12,6 +12,7 @@ describe "KeymapManager", ->
   keymapManager = null
 
   beforeEach ->
+    mockProcessPlatform('darwin')
     keymapManager = new KeymapManager
 
   afterEach ->