File name: Consider using PascalCase

### Description

Getting a flycheck message "Consider changing `3i-3.fsx` to PascalCase". The filename cannot be translated to PascalCase - doesn't make sense. This is obviously a parse error somewhere. I think. :)

### Repro steps

1. Step A
Create a file with the given name.

2. Step B
Open it with Emacs, fsharp-mode set to current major mode.

### Related information

* Operating system
Ubuntu 16.04.3 LTS
* Branch
Using fsharp-mode version "20170918.842".
* Emacs version
24.5.1
* .NET Runtime, CoreCLR or Mono Version
5.2.0.244 (Mono)
4.1 (F# compiler)