Revert "Use version.rb in gemspec"

This reverts commit 55cfee6.

Unfortunately, this doesn't work well in some places because RubyGems
depends on openssl in some code paths. This ends up with defining
OpenSSL::VERSION twice and producing warning:

	/work/ruby/ext/openssl/lib/openssl/version.rb:4: warning: already initialized constant OpenSSL::VERSION
	/work/ruby/.x86_64-linux/.ext/common/openssl/version.rb:5: warning: previous definition of VERSION was here

openssl.gemspec

@@ -1,8 +1,6 @@
-require_relative 'lib/openssl/version'
-
 Gem::Specification.new do |spec|
   spec.name          = "openssl"
-  spec.version       = OpenSSL::VERSION
+  spec.version       = "2.2.0"
   spec.authors       = ["Martin Bosslet", "SHIBATA Hiroshi", "Zachary Scott", "Kazuki Yamaguchi"]
   spec.email         = ["ruby-core@ruby-lang.org"]
   spec.summary       = %q{OpenSSL provides SSL, TLS and general purpose cryptography.}