Fix changelog for v0.6.4

[ci skip]
Add dedicated integration spec for rspec3.1
Ensure Process.widpid call happens in presence of killfork exceptions
Prepare v0.6.5
Fix dates in changelog.

[ci skip]
Use actor based parallelization
Add Subject#source_lines
Add Mutant::Tracer

* Depends on `TracePoint` thus removes 1.9 compatiblity. Do not expect
  someone is harmed by this.