Fix actor receiver to use private attr getters instead of ivars

* Ivars lead to typos being caught late. Typos on send get caught early.
Remove redundant require
Update ice_nine dependency to ~>0.11.1
Fix flay score
Prepare 0.7.0
Adjust dependencies for rspec-core
Remove completed TODO items
Add Subject#source_lines
Add Mutant::LineTrace

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