fix issue #242

I'm not sure this change will fit all platforms.
Maybe you sould add this method to the MetacelloPlatform class for GemStone and Squeak:
tempFileFor: aName suffix: aSuffixString
+ ^ '/tmp/' , aName , aSuffixString
@demarey, I think that this change needs to be moved into the class MetacelloPlatform ... the pharo tests are green, but none of the other platforms are passing ...