Add "--strict-json" flag to tools/generate-test-fixture.js

Currently tools/generate-test-fixture.js outputs JSON objects whose keys are not
double-quoted. This changes the tool to accept a `--strict-json` argument which
indicates that it should always double-quote object keys.

This is useful for piping the output of generate-test-fixture.js to other
tools that expect valid JSON.

https://code.google.com/p/esprima/issues/detail?id=461
Travis CI: Skip building on Node.js 0.6.

The new library optimist for tools/generate-test-fixture.js causes a
module install error.

https://code.google.com/p/esprima/issues/detail?id=461