Fix up libraries a bit

Remove reference to libraries in schema_cpp and list all libraries in
propert order in capnp.pyx
Add automatic build of libcapnp if it's not detected
Change travis to build with and without libcapnp pre-installed
Fix problem with detect libcapnp function
Add -fPIC flag to libcapnp build
Add set -euxo pipefail to travis setup script
Add O2 and DNDEBUG flags to libcapnp build
Remove set -u from setup_travis.sh
Change setup_travis.sh to build current release
Fix alignment issues for reading bytes
Add install flags --force-bundled-libcapnp and --force-system-libcapnp
Fix problem introduced with alignmnet fix
Fix travis tests by getting rid of pypy+BUILD_CAPNP=false build