rake is a runtime dependency

See #54, release log:

• 696a72b by @yegor256: #54 rultor fix
• 70bd5b9 by @yegor256: #54 deps easy
• 49264ad by @yegor256: todo file removed
• d5eb733 by @yegor256: rubocop 0.42.1
• 5bd623f by @yegor256: ruby 2.2.2
• d940464 by @yegor256: build fixed
• 14a659c by @yegor256: slop back to 3.6.0
• 569ede3 by @yegor256: more doc on exclude
• 9cdc46b by @yegor256: #51 deps up
• 0a622db by @yegor256: #50 exclude example
• 1137402 by @yegor256: #49 more doc about installatio...
• ceefe3f by @yegor256: badge
• aa3d068 by @yegor256: #47 zlib1g-dev

Released by Rultor 2.0-SNAPSHOT, see build log