Add Cocoapods ~> 1.8 as the gem bundle dependency

[nivanchikov]

Motivation: Even though @fbernutz already updated the Podfile.lock to Cocoapods 1.8.3 in #31, having the library version in the Podfile.lock file only is not enough for the 1.8 version to be used on any development setup. Since the Cocoapods gem is not included in the project gem bundle, simply running pod install won't automatically use the latest Cocoapods library version. E.g. my dev setup might have v1.7.5 installed and running pod install would revert the changes made in Podfile.lock.

Proposed solution This pull request adds Cocoapods ~> 1.8 as the gem bundle dependency, which allows to sync the Cocoapods gem version across different development environments. The gem version is stored to Gemfile.lock similar to Podfile.lock. The project libraries can then be installed with an always predictable Cocoapods version by running

bundle install
bundle exec pod install / update

[codecov-io]

Codecov Report

Merging #33 into master will decrease coverage by 0.7%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
- Coverage   69.85%   69.15%   -0.71%     
==========================================
  Files          13       13              
  Lines         428      428              
==========================================
- Hits          299      296       -3     
- Misses        129      132       +3

Impacted Files	Coverage Δ
...uxMovieDB/Extensions/UIImageView+MoviePoster.swift	91.8% <0%> (-4.92%)	:arrow_down:

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