Improve accuracy of build time

[jmcker]

Since the webhooks are based on commit time and include some amount of error (CI boot time, delay before pushing, etc), we could attempt to calibrate them.

An even better solution would be to remove the reliance on commit time. Perhaps something like:

curl https://raw.githubusercontent.com/symboxtra/travis-ci-discord-webhook/master/send.sh > send.sh
source ./send.sh --record-build-time
# do build and test
./send.sh success $WEBHOOK_URL

where the first command exports a variable like BUILD_START_TIME, and the second uses it to calculate build time. We could still use time-since-commit as a fallback/backwards compatibility.

Copied from jenkins-discord-webhook issue.