split tests up by shell for concurrency

More parallelism = faster tests.  Also better isolation for changes that
might only break on zsh or bash respectively.

This is defined via env variable, so someone running locally will have
all tests run sequentially as before.

.travis.yml

@@ -2,6 +2,12 @@ os:
   - linux
   - osx
 
+env:
+  - TEST_SHELLS="zsh bash"
+  - TEST_SHELLS=bash
+  - TEST_SHELLS=zsh
+  - HI=mom
+
 install:
   - ./test/support/travisci_deps.sh