Be defensive when passing potentially nil strings to Gem::Version

lib/cocoapods/command.rb

@@ -114,7 +114,9 @@ module Pod
     #
     def self.git_version
       raw_version = Executable.capture_command('git', ['--version']).first
-      match = raw_version.scan(/\d+\.\d+\.\d+/).first
+      unless match = raw_version.scan(/\d+\.\d+\.\d+/).first
+        raise "Failed to extract git version from `git --version` (#{raw_version.inspect})"
+      end
       Gem::Version.new(match)
     end