Help text has Unicode quotes in a few places, which breaks help output.
This change replaces them with ASCII quotes
Also add a test which checks the output of "snapcraft --help",
"snapcraft help topics", "snapcraft help sources", "snapcraft help
plugins" and "snapcraft help <PLUGIN>" for each plugin to make sure they
don't contain non-ASCII characters which would break the help output
when run normally
We applied a hack in travis to get a xenial LXC and run the tests in
there. Something changed and now the tests are taking an excessive
amount of time, and timing out. We no longer need this tests to run in
xenial because we are already running them in Jenkins, so we can get rid
of the hack.
