Fail loudly if VERSION cannot be read from lib

Previously the gemspec dereferenced MatchData#[] directly, so a
formatting change in lib/github-markup.rb that broke the regex
would raise NoMethodError on nil. Wrap the match in an explicit
check and raise with the file path so the failure is obvious.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Zack Koppert <zkoppert@github.com>

Author: zkoppert

github-markup.gemspec

@@ -1,5 +1,7 @@
-version = File.read(File.expand_path("../lib/github-markup.rb", __FILE__))
-              .match(/VERSION = ['"]([^'"]+)['"]/)[1]
+version_file = File.expand_path("../lib/github-markup.rb", __FILE__)
+version_match = File.read(version_file).match(/VERSION = ['"]([^'"]+)['"]/)
+raise "Could not find VERSION in #{version_file}" unless version_match
+version = version_match[1]
 
 Gem::Specification.new do |s|
   s.name          = "github-markup"