Set fresh TRAVIS_COMMIT

[gdbelvin]

Refer to the current commit hash every time.
This script results in incorrect hashes when HEAD moves after setting the
TRAVIS_COMMIT environment variable.

[codecov]

Codecov Report

Merging #1438 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1438      +/-   ##
==========================================
+ Coverage   66.36%   66.41%   +0.04%     
==========================================
  Files          54       54              
  Lines        4026     4026              
==========================================
+ Hits         2672     2674       +2     
+ Misses        961      960       -1     
+ Partials      393      392       -1

Impacted Files	Coverage Δ
core/sequencer/server.go	73.61% <0%> (ø)	⬆️
core/sequencer/trillian_client.go	61.42% <0%> (+2.85%)	⬆️

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