.travis.yml: Roll back to license-test-files d2035d9

Until [1] lands, license-test-files d19d48d will break testing (by
comparing our locally-broken XML against fixed test data).  While that
change is in flight, stick with the previous, broken test data.

An alternative approach to getting a particular commit would be to use
submodules, but:

* I expect most contributors will find this approach easier to
  manipulate, because you don't have to wrap your head around
  submodules to use it.

* The full clone (needed to ensure we still get the commit we want) is
  a price paid by GitHub (hosting the clone) and Travis (pulling the
  clone).  As long as they're both willing to pay that price, it
  doesn't seem like it's worth jumping through too many hoops to be
  more efficient.

[1]: spdx#589

.travis.yml

@@ -2,7 +2,8 @@ matrix:
   include:
     - language: java
       before_install:
-        - git clone --depth 1 https://github.com/goneall/license-test-files
+        - git clone --branch master https://github.com/goneall/license-test-files
+        - git -C license-test-files checkout d2035d9c5327e4acbde7b3d8b24501f9d5ac6438
       script:
         - make TEST_DATA=license-test-files/simpleTestForGenerator validate-canonical-match
     - language: node_js