Add missing re dependency to opam file

[arielschiavoni]

This PR adds a missing re dependency to ReasonNativeProject.opam

Context:

I followed the steps in the README and it did not work out of the box. I got an error related to a missing re dependency after running:

opam pin add -y ReasonNativeProject .

=-=- Processing actions -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=  🐫
[ERROR] The compilation of ReasonNativeProject failed at "jbuilder build -p ReasonNativeProject -j 4".

#=== ERROR while installing ReasonNativeProject.0.0.1 =========================#
# opam-version 1.2.2
# os           darwin
# command      jbuilder build -p ReasonNativeProject -j 4
# path         /Users/arielschiavoni/.opam/4.02.3/build/ReasonNativeProject.0.0.1
# compiler     4.02.3
# exit-code    1
# env-file     /Users/arielschiavoni/.opam/4.02.3/build/ReasonNativeProject.0.0.1/ReasonNativeProject-26999-ffb3fd.env
# stdout-file  /Users/arielschiavoni/.opam/4.02.3/build/ReasonNativeProject.0.0.1/ReasonNativeProject-26999-ffb3fd.out
# stderr-file  /Users/arielschiavoni/.opam/4.02.3/build/ReasonNativeProject.0.0.1/ReasonNativeProject-26999-ffb3fd.err
### stderr ###
# Error: External library "re" not found.
# -> required by "lib/jbuild (context default)"
# Hint: try: jbuilder external-lib-deps --missing -p ReasonNativeProject @install

[jordwalke]

Thank you @arielschiavoni

@jaredly did you not run into this?

[jaredly]

@jordwalke that's one annoying thing about jbuilder + opam ==> If you already have a dep installed in your opam switch, you won't get an error for leaving it out. This looks good to me!