Build fat-binaries instead of only 1.9.3 ones

Now that we have 1.8.7, 1.9.3 and 2.0.0, we can safely build
fat-binaries.
JRuby: update and lock version of RubyGems

JRuby 1.7.4 do not have RubyGems 1.8.25 with it, so an update is
required.
Use rake-compiler 0.9.0