[Jenkins] Multibranch Pipeline: Could not find a CI source for this run

# Report

## What did you do?

Run `danger-swift ci` inside a Multibranch Pipeline's Jenkinsfile sh script (Connecting to Bitbucket Server)

## What did you expect to happen?

Being able to run the Dangerfile and use a bitbucket bot account to post comments in the target PR

## What happened instead?

Although I am setting the Bitbucket Server's host, the account and password of the bot, and also verifying that CHANGE_BRANCH and CHANGE_ID are set on the environment variables, danger-swift keeps reporting that `**Could not find a CI source for this run**` and asking if I am using a CI that is not supported.

## Your Environment

* Jenkins CI
* Danger-swift version: 3.4.1
* Danger version: 10.1.0
* Dangerfile contents (just the sample one):

```ruby
import Danger
let danger = Danger()

// Pull out the edited files and find ones that come from a sub-folder
// where our app lives
let editedFiles = danger.git.modifiedFiles + danger.git.createdFiles
let editedAppFiles = editedFiles.filter { $0.contains("/App") }

// Let people bail from the CHANGELOG check
let hasSkipChangelog = danger.github.pullRequest.body?.contains("#no_changelog")
let hasSkipChangelogLabel = danger.github.issue.labels.first { $0.name == "Skip Changelog" }
let skipCheck = hasSkipChangelog ?? false || (hasSkipChangelogLabel != nil)

// Request for a CHANGELOG entry with each app change
if editedAppFiles.count > 0 && !skipCheck {
fail("Please add a CHANGELOG entry for these changes. If you would like to skip this check, add `#no_changelog` to the PR
body and re-run CI.")
}

```
* Jenkins script (on Jenkinsfile, using Pipelines):

```
node('ios-slave') {
stage('Clean Clone SCM') {
sh '''
rm -f -r *
git clone -b $CHANGE_BRANCH ssh://git@git.somewhere-it.com:7999/ti/project-ios.git
'''
}

stage('Cocoapods Update') {
withEnv(['LANG=en_US.UTF-8', 'PATH+PODS=/Users/slave/.rbenv/shims/']) {
sh '''
cd project-folder
pod update
'''
}
}

stage('Danger CI') {
withEnv(['DANGER_BITBUCKETSERVER_HOST=https://git.somewhere-it.com/', 'PATH+DANGER=/usr/local/bin/']) {
withCredentials([[
$class: 'UsernamePasswordMultiBinding',
credentialsId: 'My Bitbucket Server Credentials',
usernameVariable: 'DANGER_BITBUCKETSERVER_USERNAME',
passwordVariable: 'DANGER_BITBUCKETSERVER_PASSWORD']]) {
sh '''
cd project-ios
danger-swift ci
'''
}
}
}
}
```

* Environment Variables on Jenkins (printenv):

```
JENKINS_NODE_COOKIE=258328e8-8748-4801-88fd-ae80d386072e
SHELL=/bin/zsh
HUDSON_SERVER_COOKIE=2b6a4abc789835b8
CHANGE_BRANCH=feature/Branch_Name
STAGE_NAME=Danger CI
TMPDIR=/var/folders/__/clhhlm6d5fg7__h0syj9vmtw0000gv/T/
SSH_CLIENT=100.67.120.38 40126 22
BUILD_TAG=jenkins-iOS Danger-PR-665-9
CHANGE_URL=https://git.somewhere-it.com/projects/ti/repos/project-ios/pull-requests/665/overview
BRANCH_NAME=PR-665
OLDPWD=/Users/slave/Jenkins-agent/workspace/Branch_Name
WORKSPACE=/Users/slave/Jenkins-agent/workspace/Branch_Name
RUN_CHANGES_DISPLAY_URL=http://unconfigured-jenkins-location/job/iOS%20Danger/job/PR-665/9/display/redirect?page=changes
USER=slave
com.apple.java.jvmTask=CommandLine
CHANGE_AUTHOR=Tanabe, Alex
CHANGE_TITLE=WIP: Dangerfile.swift added
__CF_USER_TEXT_ENCODING=0x1FB:0x0:0x0
CHANGE_TARGET=develop
EVEREST_GOOGLE_MAP_KEY=AIzaSyCOCe-j1eywf596mhVfxpyhTr_TihQIkq8
JENKINS_HOME=/var/jenkins_home
PATH=/usr/local/bin/:/usr/bin:/bin:/usr/sbin:/sbin
RUN_DISPLAY_URL=http://unconfigured-jenkins-location/job/iOS%20Danger/job/PR-665/9/display/redirect
CHANGE_ID=665
PWD=/Users/slave/Jenkins-agent/workspace/ProjectName-665/repo
JOB_NAME=iOS Danger/PR-665
BUILD_DISPLAY_NAME=#9
BUILD_ID=9
JOB_BASE_NAME=PR-665
SHLVL=2
HOME=/Users/slave
EXECUTOR_NUMBER=7
JENKINS_SERVER_COOKIE=durable-ce4efe6484d2e6ca4193099e92758ca5
NODE_LABELS=ios-slave
DANGER_BITBUCKETSERVER_PASSWORD=****
LOGNAME=slave
DANGER_BITBUCKETSERVER_HOST=https://git.somewhere-it.com/
CHANGE_AUTHOR_EMAIL=****@somewhere.com
HUDSON_HOME=/var/jenkins_home
SSH_CONNECTION=100.67.120.38 40126 10.48.15.194 22
NODE_NAME=ios-slave
JOB_DISPLAY_URL=http://unconfigured-jenkins-location/job/iOS%20Danger/job/PR-665/display/redirect
BUILD_NUMBER=9
HUDSON_COOKIE=6f04de97-a6d7-46d9-b463-ecee252d4448
JAVA_ARCH=x86_64
DANGER_BITBUCKETSERVER_USERNAME=****
JAVA_MAIN_CLASS_32098=hudson.remoting.Launcher
```

Note: I am trying to hide sensitive information, but the overall structure of the information is here.