switch to clang-format-3.8 (#2547)

https://llvm.org/bugs/show_bug.cgi?id=30353 was filed for the unintended
line break between in e.g. “TAILQ_ENTRY(foo)\nbar;”.

Until that’s fixed or a workaround is known, we’ll live with line
breaks. To make it a bit easier for readers to see what’s going on, I
added extra line breaks around each such struct member/variable
definition, so that they at least visually are a single unit.

fixes #2174

i3bar/include/trayclients.h

@@ -18,5 +18,6 @@ struct trayclient {
     bool mapped;      /* Whether this window is mapped */
     int xe_version;   /* The XEMBED version supported by the client */
 
-    TAILQ_ENTRY(trayclient) tailq; /* Pointer for the TAILQ-Macro */
+    TAILQ_ENTRY(trayclient)
+    tailq; /* Pointer for the TAILQ-Macro */
 };