Migrate to cmake 2.8.12

[suntzu86]

This would be nice before OS or before we have a ton of users--changing versions early on affects fewer people. That said, this change isn't a huge deal.

Currently the cmake script requires version 2.8.9.

2.8.12 (released 10/2013) has some new features that improve the clarity of the script. In particular, it breaks out some common properties from set_target_properties() into more understandable/specialized function calls. The new functions have the advantage of making append/overwrite behavior more clear and also is more consistent with the behavior of input lists (does it need to be quoted? does it expect semicolon separation? etc).

Not a huge deal but improving readability, esp for new users, is a nice plus.

[sc932]

Travis is building on 2.8.12, is this an issue?

@garywang304 What cmake version do you use?

[gokceneraslan]

Compiles and works fine with cmake 3.3.2.