Added ruby 1.8 to .travis

Wasn't there before, but we want to support ruby 1.8
Added json_gem as a dependency for ruby 1.8 (only)
Changed "-> () {}" for "proc {}" so tests will work on ruby 1.8
Replaced json-style hashes for spaceship operator hashes

For ruby 1.8 compatibility
Stopped using skip for common test suite tests

There are two good reasons to do this:

1. skip isn't supported in ruby 1.8
2. much less verbose ouput
Ignore failures on the ruby 1.8 build

There are many failures due to ruby 1.8 hashes being unordered and
encodings being different to ruby 1.9. But at least there's a build
Removed jruby 1.8 mode from the build matrix because of ruby 1.8 test issues

At least we have mri 1.8 in the build matrix. Better than nothing
Moved the dependency for the json gem from the gemspec into the Gemfile

As @pd pointed out, putting conditional flags in the gemspec only has an
affect when packaging the gem, and does not help end users.

I've put it in the Gemfile instead, which will only help developers, but
at least bundler has the concept of platforms
Merge pull request #154 from iainbeeston/ruby_1_8_support

Add ruby 1.8 to travis