Fixed "gem build" warnings

Before:

$ gem build mediawiki_api.gemspec
WARNING:  pessimistic dependency on faraday (~> 0.9.0) may be overly
strict
  if faraday is semantically versioned, use:
    add_runtime_dependency 'faraday', '~> 0.9', '>= 0.9.0'
WARNING:  pessimistic dependency on faraday-cookie_jar (~> 0.0.6) may
be overly strict
  if faraday-cookie_jar is semantically versioned, use:
    add_runtime_dependency 'faraday-cookie_jar', '~> 0.0', '>= 0.0.6'
WARNING:  open-ended dependency on rake (>= 0, development) is not
recommended
  if rake is semantically versioned, use:
    add_development_dependency 'rake', '~> 0'
WARNING:  pessimistic dependency on rspec (~> 2.14.1, development) may
be overly strict
  if rspec is semantically versioned, use:
    add_development_dependency 'rspec', '~> 2.14', '>= 2.14.1'
WARNING:  pessimistic dependency on webmock (~> 1.17.2, development)
may be overly strict
  if webmock is semantically versioned, use:
    add_development_dependency 'webmock', '~> 1.17', '>= 1.17.2'
WARNING:  See http://guides.rubygems.org/specification-reference/ for
help
  Successfully built RubyGem
  Name: mediawiki_api
  Version: 0.1.0
  File: mediawiki_api-0.1.0.gem



After:

$ gem build mediawiki_api.gemspec 
  Successfully built RubyGem
  Name: mediawiki_api
  Version: 0.1.0
  File: mediawiki_api-0.1.0.gem

Change-Id: Ic435489b97ed6ded421260f1d15aadb9460b7a49