Fixed make distcheck to include repairer.hpp.

Review: https://reviews.apache.org/r/19784
Fixed make dist to properly extract PicoJSON.

Review: https://reviews.apache.org/r/19777
Updated CHANGELOG for 0.18.0-rc6.
Fixed libprocess Makefile to include c++11 headers.

Review: https://reviews.apache.org/r/19878
Updated CHANGELOG for 0.18.0-rc6.