Starting VRT fails if the provided API_URL ends with /

[AdamHepner]

This was lengthy investigation, but basically, in the "start" function (line 39 of visualRegressionTracker.py) you use string concatenation to build target URL. If hostname ends with "/", the result is:
https://my.url.com//builds
to which the server responds with 404 and invalid "Project not found".

Instead a urllib.parse.urljoin should've been used to avoid this type of problem (see https://stackoverflow.com/questions/1793261/how-to-join-components-of-a-path-when-you-are-constructing-a-url-in-python)

[pashidlos]

@AdamHepner thanks for your request
sure, this could be done as improvement