move cache directory from OpenBLAS.cache to .travis/OpenBLAS.cache

Moving the cache directory into .travis means that the check-generate.sh
script no longer compares the cache directory to the git repo, because
everything in the .travis/ directory is ignored.  Also, it is probably a
better place for it.

.travis.yml

@@ -34,7 +34,7 @@ matrix:
 # Cache used to persist the compiled BLAS library between CI calls.
 cache:
   directories:
-  - OpenBLAS.cache
+  - .travis/OpenBLAS.cache
 
 # Install the appropriate blas library (if any) and associated gonum software.
 install:

.travis/linux/OpenBLAS/install.sh

@@ -1,6 +1,6 @@
 set -ex
 
-CACHE_DIR=${TRAVIS_BUILD_DIR}/${BLAS_LIB}.cache
+CACHE_DIR=${TRAVIS_BUILD_DIR}/.travis/${BLAS_LIB}.cache
 
 # fetch fortran to build OpenBLAS
 sudo apt-get update -qq && sudo apt-get install -qq gfortran