docs(contribute): adding npm install to step-by-step

npm install is listed in the dependencies section of the contribute guide but
is missing from the step-by-step. This adds it as step 4.
docs(jqLite): fix typo
chore(Rakefile): skip build parallelization on Travis

Due to a infrastructure change on Travis starting JVMs in forked
processes doesn't currently work. Since we don't really care that
much about the build speed on Travis, I'm going to disable it there.

Related issue: https://github.com/travis-ci/travis-ci/issues/845