Remove version constraints to get newest gems

Removed rcov since it does not support Ruby 1.9, maybe use simplecov.
Remove rcov rake task
Refresh gemspec
Adapt to new MultiJson error handling behavior

First of all, CloudApp changed their error response to /items to always return text/plain and never application/json even if requested so I updated the stub.

And MultiJson changed their error handling behavior that tries to parse these text/plain and raises an error that is now wrapped into a CloudApp response error too.
The last commit fixes 4 pending specs too
Parse date/times automatically on behalf of Crack

HTTParty 0.8.0 and above don't use Crack (and it's very special feature of automatically parsing dates and times from JSON)
as default engine anymore and the JSON standard does not include a Date type anyways so I move some explicit parsing code
into Base.load.

Reference: https://github.com/jnunemaker/httparty/issues/100
Prepare for travis
Use syck only on Ruby < 2.0

Inspired by https://github.com/tamc/Slogger/commit/916e3e35598c09991327386256113c9340243a9a
Merge pull request #12 from cmur2/newer-gems

Updated gem dependencies, achieved Ruby 2.0 compatibility