Improve robustness, markup and parsing of build log on Travis

[heplesser]

The work on #380 and #391 revealed that the build log generated on Travis should be improved:

• Instead of relying on text or commands, introduce explicit marker lines with special format for mark the beginning and end of certain sections, e.g. ======= BEGIN VERA ======= and ======= END VERA =======. This will allow for much more robust parsing, even as the content of sections evolves.
• If the statistical analysis has found problems, the final report may contain the entire diff result. Thus, the summary becomes very difficult to read. I suggest to keep it shorter, at least not include the full diff if it is longer than a few lines.
• On rare occasions, e.g. Travis Build 841.4 the

curl https://api.github.com/repos/nest/nest-simulator/pulls/389/files

in build.sh may fail, presumably if Github cannot fullfill the request on time, leading to a

jq: error (at <stdin>:4): Cannot index string with string "filename"

In that case, the file_names variable in build.sh remains empty, no files are tested for formatting errors, and the build may pass even if the PR contained files with formatting errors. This should not happen: if the command filling file_names fails, the build should stop.

[tammoippen]

Please see #395 if it fixes most of the points.

[heplesser]

@gtrensch Given your suggestions for further improvements, I leave this issue open even after #395 is merged and leave it in your care.

[gtrensch]

This issue has been resolved by #546 and can be closed.