work around rspec `its` deprecation with rspec-its

Was seeing this deprecation message in the rspec output

    Use of rspec-core's `its` method is deprecated. Use the rspec-its gem
    instead.
updating deprecated rspec methods: be_true, be_false
expose BurstStruct::Burst#keys

expose @hash.keys through BurstStruct::Burst#keys
Merge pull request #20 from jordanstephens/burst-keys

[#19 to develop branch] Expose BurstStruct::Burst#keys