Update jenkins.py

I found a bug in the build_job function, self[jobname].invoke(params=params) should have "build_params" instead of "params" ,i.e. self[jobname].invoke(build_params=params)

Not sure where else this function is being used through out the api library,but the issue could be in other places.

Hope that helps,
S.A.
Merge pull request #123 from s-ltf/patch-1

Update jenkins.py