update CHANGELOG.md
Fix typo in README.markdown
remove special-case for 'main' from Pry.view_clip().
Revert "remove special-case for 'main' from Pry.view_clip()."

This reverts commit a56ac3f73dd375be208320b1f98c62a8cdfd5df7.

because, jruby.
cc @headius
add comment about special-case for jruby.
add Pry.main. cache its value.
update Pry.binding_for() to use Pry.main
update whereami command to use Pry.main (last one).
Merge branch 'master' into remove_mri18_support
fix 'exotic object' test on 2.1.0