Setup a regular (weekly) builds on Travis

[svetlyak40wt]

The world is changing and it will be useful to check Mito against it by using "cron" builds on Travis.

For example, this pull request's check has failed:

https://travis-ci.org/fukamachi/mito/jobs/473517349

but by looking at error:

Running a test file '/home/travis/build/fukamachi/mito/t/db/sqlite3.lisp'
> Error: Unbound variable: DBD.SQLITE3::PREPARED
> While executing: #<STANDARD-METHOD DO-SQL (DBD.SQLITE3:<DBD-SQLITE3-CONNECTION> STRING)>, in process listener(1).
> Type :GO to continue, :POP to abort, :R for a list of available restarts.
> If continued: Skip evaluation of (ros:run '((:eval"(ros:quicklisp)")(:script "/home/travis/.roswell/bin/run-prove""mito-test.asd")(:quit ())))
> Type :? for other options.
1 > 

I can confidently say that this error can't be caused by the pull. Obviously, something in cl-dbi has changed since last successful Mito's build which happened 3 months ago.