[ci] Require Clang 3.9 instead of 3.6 and 3.7
[ci] Require Clang 3.9 instead of 3.6 and 3.7

This is the first version of Clang to offer a reasonable resolution of
the C++ Core Language Defect Report 253. Earlier versions of Clang
complain about the 'hc' object from libcommute.

http://open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#253

aeantipov/pomerol,aeantipov/pomerol,aeantipov/pomerol