[Fix #176] Opened files added to the cache as if belonging to the root directory

Variable 'current-file' was refering to the file basename in function
projectile-cache-current-file.

It now refers to the relative path of the file from the project root directory.
Merge pull request #179 from jnnh/fix-opened-file-basename-added-to-the-cache

[Fix #176] Opened files added to the cache as if belonging to the root