Continue to next item if already crawled

There's nothing further we need to do if a URL has already been crawled,
so continue to the next item.
Flip 'URL exists' logic for readability

Adjust the `if` statement to avoid the negative evaluation,
keep error checking code together and separate out the positive action,
`outbound <- message`.
Continue on error
Simplify if statement
Show Travis badge only for master branch

The Travis build badge was showing as 'failing' because it represented
all branches, some of which were failing.

Change it so that it only represents the master branch, which is what we
care about.
Merge pull request #44 from alphagov/fix-tests-badge

Show Travis badge only for master branch
Merge pull request #43 from alphagov/tidy-continues

Tidy continues and simplify some `if` blocks
Ensure blacklisted seed URLs not written to disk
.