[C++] Pin clang-format version

[asfimport]

This will avoid accidental code reformatting if the user has a different version of LLVM installed.

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

_{Note: This issue was originally created as ARROW-1221. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
PR: #859

[asfimport]

Uwe Korn / @xhochy:
Issue resolved by pull request 859
#859