remove leading slash from .pyup.yml to fix bug on gitlab api

[ckleemann]

As reported in #374 the support for gitlab is broken. This is due to a change in the gitlab api. Gitlab dose not accept a urlencoded leading slash in a file path. Non urlencoded leading slashes are fine. A leading slash is used to receive the pyup.yml. The the path is then urlencoded by the gitlab library.

There is no need to for the leading slash. At this point there is no previous file request which could fail a relative file path request. So I removed the slash. Which let the api call on gitlab work again.

Fix #374

[codecov[bot]]

Codecov Report

Merging #375 into master will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #375      +/-   ##
==========================================
+ Coverage   94.19%   94.22%   +0.02%     
==========================================
  Files          11       11              
  Lines        1155     1161       +6     
==========================================
+ Hits         1088     1094       +6     
  Misses         67       67              

Impacted Files	Coverage Δ
pyup/bot.py	97.88% <100.00%> (ø)
pyup/providers/gitlab.py	85.24% <100.00%> (+0.24%)	:arrow_up:
pyup/providers/github.py	99.39% <0.00%> (+<0.01%)	:arrow_up:
pyup/requirements.py	88.46% <0.00%> (+0.06%)	:arrow_up:

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[ckleemann]

I removed the leading slash for pyup.yml on all places in bot.py. I also added the lstrip on the gitlab provider as suggested. @rafaelpivato Could you merge the pr and release a new version with the fix?

[rafaelpivato]

@ckleemann we are just waiting for #348 to make sure we are not breaking some stuff before publishing to PyPi