Upped the timeout for curl to 15 seconds to match other code

[fuddlesworth]

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The default --connect-timeout=5 argument for curl is too small and is preventing many packages from properly installing.

nathan@UNICORNVOMIT:~$ /usr/bin/curl -q --globoff --show-error --user-agent Linuxbrew/2.1.16\ \(Linux\;\ x86_64\ Ubuntu\ 18.04.3\ LTS\)\ curl/7.58.0 --fail --location --remote-time --continue-at 0 --output /home/nathan/.cache/Homebrew/downloads/38643e81048ad5dcd6482ba07097ed3e62ae22dfdb272a8187d0745c1cde60b1--sbt-1.3.3.tgz.incomplete https://sbt-downloads.cdnedge.bluemix.net/releases/v1.3.3/sbt-1.3.3.tgz --connect-timeout 5
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:--  0:00:05 --:--:--     0
curl: (28) Resolving timed out after 5552 milliseconds
nathan@UNICORNVOMIT:~$ /usr/bin/curl -q --globoff --show-error --user-agent Linuxbrew/2.1.16\ \(Linux\;\ x86_64\ Ubuntu\ 18.04.3\ LTS\)\ curl/7.58.0 --fail --location --remote-time --continue-at 0 --output /home/nathan/.cache/Homebrew/downloads/38643e81048ad5dcd6482ba07097ed3e62ae22dfdb272a8187d0745c1cde60b1--sbt-1.3.3.tgz.incomplete https://sbt-downloads.cdnedge.bluemix.net/releases/v1.3.3/sbt-1.3.3.tgz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 49.4M  100 49.4M    0     0  3221k      0  0:00:15  0:00:15 --:--:-- 11.9M

The default value also does not match the --connect-timeout value in other places show here.

nathan@UNICORNVOMIT:/home/linuxbrew/.linuxbrew$ pt connect-timeout
./Homebrew/Library/Homebrew/download_strategy.rb
440:    args << "--connect-timeout" << "5" unless mirrors.empty?

./Homebrew/Library/Homebrew/dev-cmd/pull.rb
464:    headers, = curl_output("--connect-timeout", "15", "--location", "--head", url)

./Homebrew/Library/Homebrew/utils/curl.rb
192:    "--connect-timeout", "15", "--max-time", max_time, url,

[MikeMcQuaid]

Seems reasonable! Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @MrKumaPants!