build_docs success checking is not working on Windows

[bsipocz]

The appveyor builds are now failing to notice the "build succeeded." line in the sphinx output.
The check was introduced in #223, but at the time the appveyor build was not working due to an unrelated graphiz issue.

See: https://ci.appveyor.com/project/Astropy/astropy-helpers/build/1.0.434/job/umdl56aw8a60po29

[bsipocz]

The bug is related to the line ending in windows, we either need a conditional for line 190 which is a bit hacky, or doing something more hacky a bit further down. I'll add the fix to the PR #267.