Switching to standard gem for Ruby styling

[kpumuk]

Since its inception, this gem has been using heavily customized Rubocop code style rules, based on my personal preferences and very opinionated view on how Ruby code should look like.

I think it is time to switch to a more community-approved code style, supported by the standard gem.

As it does not support Rubocop extensions, we will still use Rubocop, but apply standard rules by default (see RuboCoping with legacy: Bring your Ruby code up to Standard).

Edit: Currently, CodeClimate does not have a channel for Rubocop 1.35.1 (required by the current version of the "standard" gem). Also, CodeClimate build does not fail for some reason, but spits out this to the output:

Channel rubocop-1-35-1 not found for rubocop, available channels: ["stable", "beta", "cache-support", "rubocop-0-42", "rubocop-0-46", "rubocop-0-48", "rubocop-0-49", "rubocop-0-50", "rubocop-0-51", "rubocop-0-52", "rubocop-0-54", "rubocop-0-55", "rubocop-0-56", "rubocop-0-57", "rubocop-0-58", "rubocop-0-59", "rubocop-0-60", "rubocop-0-61", "rubocop-0-62", "rubocop-0-63", "rubocop-0-64", "rubocop-0-66", "rubocop-0-65", "rubocop-0-67", "rubocop-0-68", "rubocop-0-69", "rubocop-0-70", "rubocop-0-71", "rubocop-0-72", "rubocop-0-73", "rubocop-0-74", "rubocop-0-75", "rubocop-0-76", "rubocop-0-76-airbnb", "rubocop-0-77", "rubocop-0-78", "rubocop-0-79", "rubocop-0-80", "rubocop-0-81", "rubocop-0-82", "rubocop-0-83", "rubocop-0-84", "rubocop-0-85", "rubocop-0-86", "rubocop-0-87", "rubocop-0-88", "rubocop-0-89", "rubocop-0-90", "rubocop-0-92", "rubocop-1-70", "rubocop-1-7-0", "rubocop-1-8-1", "rubocop-1-9-1", "rubocop-1-10-0", "rubocop-1-11-0", "rubocop-1-12-0", "rubocop-1-12-1", "rubocop-1-18-3", "rubocop-1-20-0", "rubocop-1-21-0", "rubocop-1-22-2", "rubocop-1-22-3", "rubocop-1-23-0", "rubocop-1-30-0", "rubocop-1-31-0"]