release 0.15.0

- modernize rakefile and gemspec
- switch to minitest

update gemspec

Creating tag for bit-struct version 0.13.6

Creating tag for bit-struct version 0.13.5

Creating tag for bit-struct version 0.13.4

Creating tag for bit-struct version 0.13.3

Creating tag for bit-struct version 0.13.2

Creating tag for bit-struct version 0.13.1

Creating tag for bit-struct version 0.12

Creating tag for bit-struct version 0.11