Fix a wrong usage of gemspec usage, relax nokogiri & faraday_middleware requirement

[jasl]

In my experience, this usage maybe wrong

if RUBY_VERSION < "2.4.0"
  s.add_runtime_dependency("nokogiri",                "~> 1.10.4")
else
  s.add_runtime_dependency("nokogiri",                "~> 1.11.0.rc2")
end

• this piece of codes should be running in developer's PC, not end users', a proof is https://rubygems.org/gems/azure-storage-blob which shows azure-storage-blob depends on nokogiri ~> 1.11.0.rc2
• Relex the version to ~> 1.10 is enough for all versions of Rubies, the bundler can find the best match version

I found this because sparklemotion/nokogiri#2168 (comment)

In addition

I also relex faraday_middleware

[ghost]

All CLA requirements met.

[coveralls]

Coverage increased (+0.05%) to 87.08% when pulling 726fb12 on jasl:relex-nokogiri into 05ea58a on Azure:master.

[jasl]

BTW, if the PR merged, could you release a new version?

[nertzy]

Looks good to me. This is more in line with standard Ruby expectations.