Updated gemspec for the new name

elixir-theme.gemspec

 # frozen_string_literal: true
 
 Gem::Specification.new do |spec|
-  spec.name          = "elixir-theme"
+  spec.name          = "jekyll-elixir-theme"
   spec.version       = "0.4.8"
   spec.authors       = ["jacek.lebioda"]
   spec.email         = ["jacek.lebioda@uni.lu"]
 
   spec.summary       = "Jekyll theme for ELIXIR blog site"
-  spec.homepage      = "https://git-r3lab.uni.lu/elixir/elixir-theme"
+  spec.homepage      = "https://git-r3lab.uni.lu/elixir/jekyll-elixir-theme"
   spec.license       = "Apache2"
 
   spec.files         = `git ls-files -z`.split("\x0").select { |f| f.match(%r!^(assets|_layouts|_includes|_sass|LICENSE|README)!i) }