Raise error if a Message re-uses a field number.
bump version
take force_encoding as a standard feature
assume we always have ord to turn strings or integers into integers
assume the presence of bytesize and byteslice
remove unsupported rubies from travis
Merge branch 'master' into retailnext-field-reuse

Conflicts:
	test/message_test.rb
Merge pull request #47 from protobuf-ruby/bk-remove-18

Deprecate old versions of Ruby
Merge pull request #54 from protobuf-ruby/retailnext-field-reuse

Throw an error if a Message tries to reuse a field number
call the instance method that lists fields __beefcake_fields__
Merge branch 'master' into bk-fields

Conflicts:
	test/message_test.rb