Upgrade CircleCI

CircleCI 1.0 is sunsetting as of 8/31/2018. This release upgrades to CircleCI 2.0. It will also trigger the upgrade to heroku-16 stack.

Also:

• Update some dependencies with vulnerabilities.
• Update ruby to 2.5.0

Rename primary controller

To get more in line with the rest of the app, rename the Math24Controller to GetTo24Controller.

New Math24 Syntax

Update app to use new Math24 syntax. Also:

*Add specs for Math24Controller
*Change rspec output to use documentation format
*Update gems and add some groupings to Gemfile
*Delete unused lib and bin files
*Add logging files to .gitignore
*Delete old gem and gemspec files (brought over in extraction from Math24 gem)
*Delete old test files for gem
*Delete unused controller action 'post /problem'
*Delete lots of comments
*Remove dependency on Math24 last_answer instance variable
*Fix some whitespace errors
*Delete capture_stdout method from spec_helper
*Other minor refactoring

Extract Sinatra app from gem code

Merge pull request #1 from randallreedjr/bug/math24-integer-division

Update to math24 2.0.1