Upload pull request builds to transfer.sh

[WithoutPants]

Adds a script to upload the binaries built from a pull request to transfer.sh. The URL is output in the build log. By submitting this pull request, I'm hoping that it will test successfully.

I'm not sold on the idea of uploading three large binaries for every build of every pull request, but this (hopefully) shows that it can be done.

Related to #139

[WithoutPants]

Build ran for 13 minutes, 12 seconds. Average build prior to this change is around 8 minutes. Looks like most of the extra time is spent performing the cross compile. Upload is only taking around 15 seconds.

16.45s$ if [ "$TRAVIS_PULL_REQUEST" != "false" ]; then sh ./scripts/upload-pull-request.sh; fi
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 30.1M  100    35  100 30.1M      8  7357k  0:00:04  0:00:04 --:--:-- 7622k
stash-osx uploaded to url: https://transfer.sh/muMhR/stash-osx
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 43.9M  100    39  100 43.9M      6  7133k  0:00:06  0:00:06 --:--:-- 9196k
stash-win.exe uploaded to url: https://transfer.sh/Tartr/stash-win.exe
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 31.8M  100    37  100 31.8M      6  5622k  0:00:06  0:00:05  0:00:01 7670k
stash-linux uploaded to url: https://transfer.sh/tTfP5/stash-linux

[Leopere]

It's not ideal uploading for every build but the reason I suggested Transfer.sh is that it disposes of the files after 30 days so we're not cyber littering.

[Leopere]

One of the things we should do is add a .gitignore for a .dev_env file which would contain the traefik.sh upload of the binary spat out by the dev builds. This way they can test straight away locally.

[Leopere]

The idea here would be to allow someone with a local copy of the repository to be able to test their PR's that are tested on TravisCI locally within Docker or whatever they use.

[bnkai]

It seems useful for user beta testing and the files get deleted anyway , i would say it doesn't hurt to have it...

[Leopere]

now we just need to resolve the travis.yml issue and then I will merge it.