Fix compilation with clang 4.x compiler - capnp error

[breznak]

I'd like to compile our codebase with clang++ 4.0 which is the latest stable release now.

The code fails on an error in capnp dependency:

Scanning dependencies of target nupic_core_solo
[ 29%] Building CXX object src/CMakeFiles/nupic_core_solo.dir/nupic/proto/ArrayProto.capnp.c++.o
In file included from /mnt/store/devel/HTM/nupic.core/build/scripts/src/nupic/proto/ArrayProto.capnp.c++:4:
In file included from /mnt/store/devel/HTM/nupic.core/build/scripts/src/nupic/proto/ArrayProto.capnp.h:7:
In file included from /mnt/store/devel/HTM/nupic.core/build/scripts/ThirdParty/Install/include/capnp/generated-header-support.h:31:
/mnt/store/devel/HTM/nupic.core/build/scripts/ThirdParty/Install/include/capnp/layout.h:129:65: error: value of type 'bool' is not implicitly convertible to
      'capnp::Kind'
template <typename T, bool b> struct ElementSizeForType<List<T, b>> {
                                                                ^

This is a known error already fixed upstream:
capnproto/capnproto#433

It should be fixed in 0.5.3.1 release, latest current stable is 0.6. We use pycapnp 0.5.12 from pypi that should ship with capnp 0.5.3.1.

But the error still remains?
Reported upstream
capnproto/pycapnp#152