travis: Enable build with default Linux image Bash

Since I'm paranoid that we might need to resolve an issue specific to
4.3.11(1)-release (or whatever the Bash version is in the stock Travis
Linux image). Also, I didn't want to hold up sstephenson#116 any longer with more
tiny requests.

.travis.yml

@@ -4,6 +4,7 @@ os:
 - linux
 
 env:
+  - BASHVER=
   - BASHVER=3.2
   - BASHVER=4.0
   - BASHVER=4.1
@@ -20,7 +21,7 @@ services:
 
 script:
 - |
-  if [[ "$TRAVIS_OS_NAME" == 'linux' ]]; then
+  if [[ "$TRAVIS_OS_NAME" == 'linux' && -n "$BASHVER" ]]; then
     docker build --build-arg bashver=${BASHVER} --tag bats/bats:bash-${BASHVER} .
     docker run -it bash:${BASHVER} --version
     time docker run -it bats/bats:bash-${BASHVER} --tap /opt/bats/test