Revert "Add json gem as a runtime dependency for Ruby 1.9.2"

This reverts commit 71ad28d77749eccbbe52034c7909bdfd8f266a8f.

Conflicts:
	rubocop.gemspec
Revert "Test against Ruby 1.9.2 on Travis CI"

This reverts commit 9d90af624c9c45777359124e3bd497f75f84f69e.
Bump required Ruby version to 1.9.3
Update README and changelog
Merge pull request #1194 from yujinakayama/drop-support-for-ruby-1.9.2

Drop support for Ruby 1.9.2