Doesn't build on Ubuntu 15.04 about curlcpp

[JosephP91]

@tpoechtrager I created an Ubuntu 15.04 virtual machine to reproduce your development environment and errors. Checking on libcurl website, I see that many of the not found constants, like CURLOPT_PINNEDPUBLICKEY, has been added in libcurl 7.41, while Ubuntu has libcurl 7.38 installed. Downloading libcurl 7.41, compiling it and installing it, has solved the problem and I have been able to compile curlcpp.

[tpoechtrager]

Upgrading isn't always the best solution. Why don't you #ifdef them out?
Also this seems to be related to #32.

[susnux]

I think handling this like #32 would be a good idea, anyway CURLOPT_PINNEDPUBLICKEY was added in 7.39 ;-)

[JosephP91]

@susnux right, in the 7.39. My mistake :)

[susnux]

I committed a fix for this bug. However this will only will bring backwards compatibility until 7.38.
If this should work with all curl version back to 7.28 there are few more macros which have to put in #if #endif. So @JosephP91 should it be backwards compatible? I think it is not much more work.

[JosephP91]

Mmm, yes, it should be!