update julia, make it portable accross cpu architectures

* Remove old julia versions except for 0.2.1
* Make 0.3.2 the default
* Build julia with `JULIA_CPU_TARGET=core2`. This gets rid of the impurity that the build output would be different on different build machines, eg. important when copying closures / using nixops. This is the same way the official julia binary releases are built. The actual JIT code generation is unaffected by this and is still optimized to the architecture julia is running on.

@7c6f434c could you please take a look whether you are ok with this?