Fixes #2535 - Use latest Firefox on Travis

[miketaylr]

This PR fixes issue #2535.

Proposed PR background

This change makes Travis use the latest stable Firefox, rather than a hard coded 58.

It's possible this causes failures, depending on the speed of geckodriver breaking changes or updates.

[TravisBuddy]

Travis tests have failed

Hey @miketaylr,
Please read the following log in order to understand the failure reason.
It'll be awesome if you fix what's wrong and commit the changes.

1st Build

npm run test:js -- --reporters="runner" --firefoxBinary=`which firefox`

> webcompat@ test:js /home/travis/build/webcompat/webcompat.com
> node ./tests/functional/_intern.js "--reporters=runner" "--firefoxBinary=/home/travis/firefox-latest/firefox/firefox"

Error: 
        ======================================================
        Intern checkServer Connection Error: Error: connect ECONNREFUSED 127.0.0.1:5000
        ======================================================
        
  at ClientRequest.<anonymous>  <tests/functional/lib/setup.js:55:9>
  at emitOne  <events.js:116:13>
  at ClientRequest.emit  <events.js:211:7>
  at Socket.socketErrorListener  <_http_client.js:387:9>
  at emitOne  <events.js:116:13>
  at Socket.emit  <events.js:211:7>
  at emitErrorNT  <internal/streams/destroy.js:64:8>
something bad happened inside intern.run()
{ Error: 
        ======================================================
        Intern checkServer Connection Error: Error: connect ECONNREFUSED 127.0.0.1:5000
        ======================================================
        
    at ClientRequest.<anonymous> (/home/travis/build/webcompat/webcompat.com/tests/functional/lib/setup.js:55:9)
    at emitOne (events.js:116:13)
    at ClientRequest.emit (events.js:211:7)
    at Socket.socketErrorListener (_http_client.js:387:9)
    at emitOne (events.js:116:13)
    at Socket.emit (events.js:211:7)
    at emitErrorNT (internal/streams/destroy.js:64:8)
    at _combinedTickCallback (internal/process/next_tick.js:138:11)
    at process._tickCallback (internal/process/next_tick.js:180:9) reported: true }
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! webcompat@ test:js: `node ./tests/functional/_intern.js "--reporters=runner" "--firefoxBinary=/home/travis/firefox-latest/firefox/firefox"`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the webcompat@ test:js script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /home/travis/.npm/_logs/2018-06-29T18_08_35_072Z-debug.log

[karlcow]

Let's go!