Add String#delete_prefix and delete_suffix benchmarks

[bdewater]

No description provided.

[Arcovion]

Build is failing due to a bug, if you wouldn't mind amending your last commit to:

- rvm: 2.5.1
  before_install:
    # https://github.com/travis-ci/travis-ci/issues/8978#issuecomment-354036443
    - gem update --system

According to travis-ci/travis-ci#8978 (comment) that should fix it!

[bdewater]

Thanks @Arcovion !

[Arcovion]

Whoops. I've never actually modified travis.yml, I guess it needs to be done like:

matrix:
  include:
    - rvm: 2.5.1
      before_install:
        # https://github.com/travis-ci/travis-ci/issues/8978#issuecomment-354036443
        - gem update --system

If I'm reading this correctly, it'll run the gem update --system only for 2.5.1; you can then remove the other mention of - 2.5.1 above.

OR we can simply add 2.5.1 to the allowed_failures list for now and remove it once the bug is fixed:

 matrix:
   allow_failures:
     - rvm: 2.5.1

[Arcovion]

Thanks! 💖