Remove unsuitable gem dependencies

[ronaldtse]

  spec.add_dependency "mathml2asciimath"
  spec.add_dependency "nokogiri", "~> 1.13"
  spec.add_dependency "oscal", "~> 0.1.1"
  spec.add_dependency "parslet"
  spec.add_dependency "premailer", "~> 1.11.0"
  spec.add_dependency "word-to-markdown"
  spec.add_dependency "base64"
  spec.add_dependency "thor"

• [ ] Replace mathml2asciimath with plurimath
• [ ] Remove premailer and word-to-markdown gems

[hmdne]

plurimath: #85