[tor-bugs] #25714 [Core Tor/Tor]: switch Travis to using Rust stable

[Tor Bug Tracker & Wiki]

#25714: switch Travis to using Rust stable
------------------------------+--------------------------------
     Reporter:  catalyst      |      Owner:  (none)
         Type:  task          |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.3.4.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  tor-ci, rust
Actual Points:                |  Parent ID:  #25550
       Points:                |   Reviewer:
      Sponsor:  Sponsor3      |
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 Right now our Travis use Rust nightly instead of stable.  Now that we are
 standardizing on a stable Rust (see #24765), we should have Travis use
 stable so we catch accidental uses of nightly features.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/25714>
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