Move to clang-format-6.0 and cleanup config

[tautschnig]

As we are moving to clang-6.0 in other parts of the Travis configuration we
might as well use clang-format-6.0 in line with this. Also clean up the script
that used to ack around paths by just specifying the binary to use on the
command-line.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a My contribution is formatted in line with CODING_STANDARD.md.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[thk123]

Might be worth running it across a folder as I seem to remember there was some undesirable consequences (hence using an old version in the first place).

[tautschnig]

I did run it across a few files and it seemed to me that any changes suggested were perfectly in line with our standards. If anyone knows files that should definitively be completely clang-format clean let me know...

[allredj]

Passed Diffblue compatibility checks (cbmc commit: ac3cb0f).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/87754821

[chrisr-diffblue]

@tautschnig It looks like this PR has caused the clang-format job on travis to start failing due to apt dependencies. CI still looks green because the clang-format job is 'optional', but the clang-format job is red.

[tautschnig]

Yes, I have noticed this last night as well. Debugging this is the next item on my to-do list, just finishing up on PR.

[tautschnig]

Fixed in #3173.