[release] 1.2.1 🎉

[eonu]

Major changes

• Add support for saving PDFs with Paper#save (see Could I download pdf with arx? #81). (#90)
• Change gem Ruby version specifier from ~> 2.5 to >= 2.5. (#86)

Minor changes

• Remove arXiv logo from README.md. (#87)
• Use URI.open instead of Kernel.open (see Use URI.open instead of Kernel.open (after 2.7) #82). (#86)
• Add CI rubies 2.7 and 3.0 (see Add more CI rubies #84). (#86)
• Remove unnecessary arXiv information from README.md (see Remove unnecessary arXiv information from README.md #83). (#86)
• Remove system gem update directive in .travis.yml. (#78)
• Remove version specifier for bundler gem in arx.gemspec. (#77)
• Update specs using paper 1710.02185 to account for revision and acceptance by journal (congratulations!). (#76)
• Update email address in LICENSE.md. (#73)