[ycm] Switch from gnu++11 to gnu++14

The chromium style guide allows the use of c++14 features
(https://chromium-cpp.appspot.com/).
Thus ycm should use c++14 by default.

R=bmeurer@chromium.org

Change-Id: I6eca99931d117ad57213e8c6e18e75d83b7fd675
Reviewed-on: https://chromium-review.googlesource.com/789873
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49624}

.ycm_extra_conf.py

@@ -42,7 +42,7 @@ import sys
 # Flags from YCM's default config.
 flags = [
 '-DUSE_CLANG_COMPLETER',
-'-std=gnu++11',
+'-std=gnu++14',
 '-x',
 'c++',
 ]