serrano v1.4

• Moved to faraday > v2. There's no user facing changes here, but let me know if any issues arise (#172)
• PR by @LocoDelAssembly fixes Serrano.content_negotiation(format: "citeproc-json") by having it return nil instead of Resource not found when no DOI is found, so that the output is more compatible with flows that use serrano to create JSON (#169)
• PR by @xuanxu adds REXML as a runtime dependency (#159)