Fix Travis builds and add new rake tasks

[spovich]

Add Rakefile (run rake -T to see new rake options)

• Add ‘rake’, ‘simplecov’, and 'travis-lint' gems as development dependencies.
• Add travis build status and gem version badges to README
• Add bundler/gem_tasks to Rakefile for gem releases (adds rake tasks)
• Add rake coverage - run specs/simplecov
• Add rake spec - run specs (like rspec) - needed for travis
• Add rake validate - validate travis-ci config
• remove deprecated treat_symbols_as_metadata_keys_with_true_values in spec_helper (defaults to true)

[spovich]

@jlberglund @kshnurov if either of you have a few moments, can you review this PR? Thanks!

[jlberglund]

@spovich I don't have much experience with Travis (yet), but it looks like the only notable change outside of that is the removal of _treat_symbols_as_metadata_keys_with_truevalues. Anyways, looks good from my view.

[spovich]

@jlberglund yes, treat_symbols_as_metadata_keys_with_true_values was deprecated by rspec, so removing it cleans up the test run output. Thanks!