Update copyright to 2018.

[iphydf]

This change is 

[codecov]

Codecov Report

Merging #1127 into master will increase coverage by <.1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1127     +/-   ##
========================================
+ Coverage    82.4%   82.5%   +<.1%     
========================================
  Files          81      81             
  Lines       14415   14415             
========================================
+ Hits        11891   11897      +6     
+ Misses       2524    2518      -6

Impacted Files	Coverage Δ
toxav/video.c	67.4% <ø> (ø)	⬆️
toxcore/list.c	93.1% <ø> (ø)	⬆️
toxcore/TCP_client.c	64.3% <ø> (ø)	⬆️
toxav/audio.c	65.7% <ø> (ø)	⬆️
toxcore/network.c	83.4% <ø> (ø)	⬆️
toxcore/ping_array.c	91.9% <ø> (ø)	⬆️
toxcore/TCP_connection.c	78.2% <ø> (ø)	⬆️
toxcore/util.c	86.2% <ø> (ø)	⬆️
testing/misc_tools.c	55.1% <ø> (ø)	⬆️
toxcore/group.c	71.7% <ø> (ø)	⬆️
... and 23 more

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[sudden6]

Reviewed 85 of 85 files at r1.
Reviewable status: 0 of 1 approvals obtained