switch from Travis to Circle CI configuration

[elrayle]

• update contributing and readme to ref circle ci
• do not pin bundler
• add rspec_junit_formatter (tests fail to run at all without this)
• pin rdf and rdf-vocab to allow LoosePropertySelector to continue to function

Need further exploration to determine what changed in rdf 3.1.2 and rdf-vocab 3.1.2 that causes the test for LoosePropertySelector to fail.