docs/quickstart.rst: swap two paragraphs to make it more likely a user will report...

docs/quickstart.rst: swap two paragraphs to make it more likely a user will report import and versioning errors from 'bin/tahoe --version' if there are any, before trying to run tests. Also, change 'python setup.py test' to 'python setup.py trial' to avoid a rebuild that might change the dependencies (if setuptools decides to throw a spanner in the works).