Bump version to gdbm-2.0.0.beta1

* Merge from ruby/gdbm.
  * gdbm gem is already registered in rubygems.org. I skip shipped
    version of 0.x and 1.x. Therefore We need to use 2.x.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58502 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/gdbm/gdbm.gemspec

@@ -2,8 +2,8 @@
 
 Gem::Specification.new do |spec|
   spec.name          = "gdbm"
-  spec.version       = "0.0.1"
+  spec.version       = "2.0.0.beta1"
-  spec.date          = '2017-02-24'
+  spec.date          = '2017-04-28'
   spec.authors       = ["Yukihiro Matsumoto"]
   spec.email         = ["matz@ruby-lang.org"]