Update the notes

doc/travis_notes.txt

@@ -3,10 +3,19 @@
   http://lint.travis-ci.org/ is recommended elsewhere but does not give helpful
   error reports.
 - Make sure all of your "-" lines start on the same column
-- Use shell scripts for `before_install` and `script` or any part that
+- Use bash scripts for `before_install` and `script` or any part that
   has conditional statements
+   - Make sure they are "executable" (chmod +x)
+   - Use the following header:
+
+   ```
+     #!/usr/bin/env bash
+     set -ex
+   ```
+
 - Use `travis_retry` before a command to have it try several times before
-failing (useful for installing from third party sources)
+failing (useful for installing from third party sources).  Note that this
+command is not available within a bash script.
 - Feel free to cancel a build rather than waiting for it to go to completion
   if you have made a change to that branch.
 - A VM with 64bit Ubuntu 12.04 is a huge help for debugging.