Fix: Versions.prerelease? must skip metadata

A version number such as `1.0+meta` was considered a prerelease
because the metadata contains a letter. We must ignore the metadata
part.

Author: ysbaddaden

src/versions.cr

@@ -150,7 +150,11 @@ module Shards
     end
 
     def self.prerelease?(str)
-      str.each_char.any?(&.ascii_letter?)
+      str.each_char do |char|
+        return true if char.ascii_letter?
+        break if char == '+'
+      end
+      false
     end
 
     protected def self.without_prereleases(versions)

test/versions_test.cr

@@ -10,6 +10,10 @@ module Shards
       assert Versions.prerelease?("1.0.alpha")
       assert Versions.prerelease?("1.0.0-rc1")
       assert Versions.prerelease?("1.0.0-pre.1.2.x.y")
+
+      assert Versions.prerelease?("1.0.0-pre+20190129")
+      refute Versions.prerelease?("1.0+20190129")
+      refute Versions.prerelease?("1.0+build1")
     end
 
     def test_compare