Force TAP output from Bats on CI

In Travis CI environment, Bats thinks it's outputting to an interactive
terminal, so it switches to "pretty" format and ANSI escape codes which
don't look well in the final output.

.travis.yml

@@ -1,4 +1,4 @@
 install: git clone https://github.com/sstephenson/bats.git
-script: bats/bin/bats test
+script: bats/bin/bats --tap test
 # skips unnecessary Ruby-specific setup
 language: c