travis: add post_install hook compiling the examples, test on 4.05.0 and 4.06.0

[hannesm]

as an update for #85

[hannesm]

File "examples/hello_async.ml", line 1, characters 5-13:
Warning 3: deprecated: module Core.Std
[since 2017-02] Use Core. The Std sub-module is no longer needed
File "examples/hello_async.ml", line 2, characters 5-14:
Error: Unbound module Async.Std

seems like your hello_async needs some updating..

[hannesm]

oh, and btw, I tried only a jbuilder build ..hello_lwt.exe, but this already asked me to install async. is there a way to have this only needing lwt and cohttp-lwt-unix (I worked around this by manually removing the last block of the examples/jbuild file)?

[hannesm]

(I commented the hello_async example out for now to make travis happy, all green now) 😄

[seliopou]

Made #88 to address the build error for the async example. I'll merge that and then this should be fine to merge without commenting out the async example.

[hannesm]

@seliopou ok, force-pushed this branch after git reset --hard HEAD~1, thus you can restart travis and merge once #88 is in :)

[seliopou]

Thanks!