bumps version to 0.10.0.pre8
Don't mkdir server/sources/dylib in build_server

Rake :build_server was incorrectly making a 'dylibs'
directory in the calabash-ios-server directory which
caused the next $ make dylibs to fail with:

make: `dylibs' is up to date.

The :build_server task should make _no_ directories
in the server sources expect through calling 'make'.