Fix case in a couple of includes (Marlin.h, Configuration.h).

This fixes Ultimaker2Marlin build on case-sensitive file systems like one usually has on Linux.
Merge pull request #20 from krasin/fix-includes-case

Fix case in a couple of includes (Marlin.h, Configuration.h).
Merge branch 'master' of https://github.com/Ultimaker/SecretMarlin