Updated French to 1.8.0

Updated the French language pack to 1.8.0 :
- translated 'cannot_update_extensions_disabled'
- 2 minor changes
- bumped version to 1.8.0
Reverted back Mercredi to "M" (single letter)

Reverted back line 677 "Mercredi" to "M" instead of "Me" for the one
letter word.
Merge pull request #5 from ebeauchamps/master

Updated French to version 1.8.0