Fix META file

[samoht]

Note: it seems that the order of elements matters here. /cc @dbuenzli

[dsheets]

Matters how? The previous content built fine for me with ocamlfind 1.6.2.

[samoht]

It builds fine, but when you #require "github-hooks.unix in a toplevel you have a problem if the META file doesn't list the packages in the right order.

[dbuenzli]

What kind of problem ?

[samoht]

The github package has 2 implementation github.unix and github.jsoo. github-hooks depends on github, but github-hooks.unix depends on github.unix. So github-hooks-unix has to lad github.unix before trying to load github-hooks.