Travis: Cache MOM6-examples

- Return to using MOM6-examples caching. Recursively cloning everything
  adds a few minutes to the through put and should be unnecessary.

.travis.yml

@@ -22,7 +22,6 @@ cache:
 # Install tools and clone the configurations repository
 before_install:
  - git clone https://github.com/adcroft/house_cleaning.git
- - rm -rf MOM6-examples/
  # This line clones MOM6-examples when there is no cache
  - test -f MOM6-examples/README.md || (rm -rf MOM6-examples && git clone https://github.com/NOAA-GFDL/MOM6-examples.git)