Fix a bug introduced by #146 in ChromeFinder.

Re-adding a fix which I had accidentally reverted during the final force push. The fix updates the location methods in the ChromeFinder to return nil if a valid binary is not found.

lib/webdrivers/chrome_finder.rb

@@ -46,6 +46,8 @@ def win_location
             return option if File.exist?(option)
           end
         end
+
+        nil
       end
 
       def mac_location
@@ -59,6 +61,8 @@ def mac_location
             return option if File.exist?(option)
           end
         end
+
+        nil
       end
 
       def linux_location
@@ -71,6 +75,8 @@ def linux_location
             return option if File.exist?(option)
           end
         end
+
+        nil
       end
 
       def win_version(location)