Fix: decoding error + gemname search pattern with rubygems > 3.0

• Fix decoding error in Python2.

  Use subprocess.check_output with universal_newlines option to prevent decoding errors.

• Store mocked gem dependency calls return in files rather than in a dictionnary

• Fix gems constraints detection failures by extending the regex to also search for symbols < and ~> in the second constraint.

• Fix: make gem_search_pattern compatible with rubygems >= 3.0

  Use search pattern ^gemname$ for rubygems >= 3.0 instead of /^gemname$/

• Extend gem constraints regex to handle version exclusion ( != symbol)