Fix bson_ext dependency in gemspec

bson_ext is not only required when the platform is ruby, to avoid
installation problems under jruby
Added json gem to server.rb.
Adding note about implemented spec version [ci skip]
Fix issue #17 - Honour token `expires_at` value
Merge branch 'fix-token-expiration'
Fix README according to recent changes (access token expiration).
Version bump to 2.7.0
assertion types can provide callbacks
Update README
Version bump to 2.8.0
Ruby formatting [ci skip]
Merge pull request #24 from biilmann/jruby-compatibility

JRuby compatibility
Reverted pull request #24
Added bundler and doc.
Merge pull request #32 from spk/bundler

Added bundler and doc.
Removed .rvmrc
add server test
Merge pull request #38 from mulderp/improve_tests

add server test
Merge remote-tracking branch 'upstream/master' into membase_support