Frege embeds a timestamp, killing reproducibility

in compiled fr to .java you see an annotation beginning like this:
```
@Meta.FregePackage(
source="test/hello0.fr", time=1504471291378L, jmajor=1, jminor=8,
imps={
```

We need a constant timestamp, or else every time we change any dependency we recompile the whole graph (needlessly it seems).

There should be an option in frege to not do this (and there may be).