travis-ci: Use = not : when writing ~/.buildman

Travis CI seems to be confused when there's a colon in an echo command,
and this is currently worked around using a variable that contains the
text we want to echo. Use = syntax instead so that we can remove the
work-around; it's rather confusing until you find out what it's for.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Heiko Schocher <hs@denx.de>