Fix two bugs of changelog checker
Fix two bugs of changelog checker

- "To be released" is usually appeared in CHANGES.rst, not README.rst.
- As `git show` doesn't work with `TRAVIS_COMMIT_RANGE` value when
  amended commit/rebased commits are pushed, it'd better to refer to
  `TRAVIS_PULL_REQUEST_SHA`/`TRAVIS_TAG`/`TRAVIS_COMMIT` for certain
  cases instead.

[changelog skip]

spoqa/nirum-python,spoqa/nirum-python