ci: skip whitespace checking if running CI for a specific tag
ci: skip whitespace checking if running CI for a specific tag

Checking the diff against $TRAVIS_BRANCH fails if running CI builds for
a certain tag. Therefore skip whitespace checks if $TRAVIS_TAG is set.

Fixes: GH-1724
Fixes: c0746b334d73ebd6cfa55f6e888cd2343d1a29f0
Signed-off-by: Jonas Witschel <87a164e5d00d5dcdd001e37b34ea8bcd3fc3a172@gmx.de>

01org/tpm2.0-tools,01org/tpm2.0-tools,01org/tpm2.0-tools