constraint ocurl version to 0.7.7

[ivg]

It looks like that ocurl.0.7.8 doesn't compile on Ubuntu Precise,
although it works fine at least on the Trusty.

[hcarty]

This may be worth submitting upstream - several other libcurl features are already wrapped behind version checks. Something new may have been introduced and be in need of such a check.

[ivg]

@hcarty, yep, but I doubt, that I will have enough time to do this in the nearest future. Also, the opam packaging of the ocurl can be done better, with a proper conf-curl that will be versioned, so that we can explicitly constraint ocurl versions with the curl versions.

[hcarty]

Do you mind if I submit an issue to the ocurl project bringing up the 12.04 compatibility issue, linking to this PR?